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>> on the night of september 26th, 2014, students from a small school in the mexican state of guerrero were driving through the city of iguala...when they were stopped by police...and then fired upon 43 of them were never seen again

>> since the students were taken, mexico has been consumed by protests. but the story goes deeper than the 43. tens of thousands of people have gone missing in mexico...as the government struggles to battle cartels. fault lines came here - before and after september's attack on the students - to try to find out why so many people are disappearing in mexico.

>> the school the missing students attended is here, the raul isidro burgos rural college of ayotzinapa - where they were training to become teachers. t's part of a network of schools known as the normales - or normalistas... ...founded in the aftermath of mexico's revolution to provide teachers for the country's poorest communities.

many of the students who were taken in september had just begun their first year at ayotzinapa. the classrooms they should be in...now serve as both a memorial and a base for the parents as they continue to search for their sons. mario, who lives nearly 10 hours away by bus, came here the day after the attack, when he found out his son cesar manuel, was among the missing students.

the last time mario spoke to his son was the day of the attack. students told us that they

were preparing to travel to mexico city that day 3...to commemorate the killings of students by the army and police in 1968, something done annually. david said he was among the students headed to iguala that night, where they hoped to find more buses to get to mexico city.

in this video from that night, filmed by a student on his cell phone, the students are surrounded on either side, unable to escape. an ambulance arrives at one point - but is not allowed in.

omar says he and other students then drove to iguala to find out what happened and help their friends.

by the time they arrived, the police had left - and the attack seemed to be over.

by the time the shooting was over, six civilians were dead including three students - and 43 others were missing. >> with the exception of one student - alexander mora - neither the students nor their remains have been found.

>> we're here to fully get into the nuances of everything that's going on, not just in this country, but around the world. >> ...as if there were no cameras here, would be the best solution. >> this goes to the heart of the argument >> to tell you the stories that others won't cover. how big do you see this getting? getting the news from the people who are affected. >> people need to demand reform... >> we're here to provide the analysis... the context... and the reporting that allows you to make sense of your world. >> ali velshi on target only on al jazeera america

in the months that followed the students disappearance, dozens were arrested, including the mayor of iguala - jose luis abarca - and his wife - who officials say ordered the attack on the students... ...allegedly believing they were going to disrupt an event they were holding that night - an allegation the students deny.

after taking the boys off the buses, investigators say police piled them into the backs of their trucks and handed them over to a local cartel that had close links to the mayor and his wife. the case of the students ...is not an isolated one in mexico according to the government's own reports, over 22,000 people have gone missing in mexico since 2006, though human rights groups believe that number to be far higher. it's been three and half years since juana last heard

from her daughter, brenda damaris - here it says i'm still alive.... find me it was late on a summer night when she called to say she'd been in a car accident, just outside the city of monterrey. the family told us that as they were talking to her, they heard a local policeman tell her to turn off her cell phone.

since damaris went missing, no one has been prosecuted for her disappearance, leaving juana and the family to continue searching by themselves.

this is the case across mexico, families left waiting...not knowing if their loved ones are alive...or if they're dead. kidnappings are a source of income for cartels, it's a way for them to extort civilians - but also to force people to work for them.

with little help from officials, families of the disappeared across mexico have organized networks to support each other and try and help advance each other's cases.

with so few meaningful investigations, one of the most important questions - why was this person taken - is rarely, if ever, answered. federico mastrogiovanni is a journalist who's been researching the issue

of disappearances over the past few years. >> why do you think there are so few investigations around disappearances here in mexico? >> every saturday night. >> i lived that character. >> go one on one with america's movers and shakers. >> we will be able to see change. >> gripping... inspiring... entertaining. no topic off limits. >> 'cause i'm like, "dad, there are hookers in this house". >> exclusive conversations you won't find anywhere else. >> these are very vivid, human stories. >> if you have an agenda with

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>> in november, about a month after the 43 students disappeared, mexico's attorney general said he believed they had been killed by cartel members... the bodies had been burnt, he said, and what was left had been put into plastic bags and tossed into a river just outside of iguala, and it would be hard to identify or match the charred remains through dna. with so many disappearances in this region, and potential dumping and burning sites, it may be impossible to ever get a full picture of what happened.

in the search for the students - carried out by officials as well as people from the community - one mass grave after another was discovered, scattered through the hills. miguel angel is part of one of guerrero's community police forces, formed in response to rising violence and local corruption - and the government's inability to curb it. they want to know if the remains they've found here could belong to the students...or to others that have gone missing in the area.

in the brush and trees, there are chunks of tire rubber and empty bottles of gasoline miguel told us that when they find sites like this, they notify the attorney general's office...but that it often takes as long as 2 days for government officials to come.

it's not just miguel that doubts the government's version though - independent forensic experts from argentina working on behalf of the parents say there are too many discrepancies...and not enough scientific evidence to prove the government's theory. the lack of trust in the ayotzinapa case isn't unusual. while there are some cases when remains in mass graves have been identified, even when they have been, it's often left families with more questions than answers.

a year after brenda damaris disappeared, local officials told juana that they had found her remains in a clandestine grave. >> all of these bodies were just piled together and thrown in a bag...you can see another skull, two skulls. but their belief that local police were involved in her disappearance - along with other discrepancies - made the family question if the remains they were given actually belonged to their daughter. her doubts led her here and to franco mora, a peruvian forensic doctor - he's going to exhume the remains the state gave the family.

the remains lie in a grave, marked by a cross with no name. this is the first time an independent forensic test is being carried out to verify the states work in a disappearance in mexico juana told us she became suspicious after officials suggested that she cremate the bones immediately. the family placed the remains given to them by the state in a communal plot, not wanting to bury them in the family's section until they are certain it is her.

the team digs for nearly 2 and a half hours - until they reach the small gray box that holds the bones. the next step will be comparing the remains to dna from the family, though the results won't be known for months.

>> the students' disappearance has been linked to the corruption of not just of the mayor of iguala, but also the city's police force... ...a problem common throughout mexico, where cartels have been able to buy off local police in exchange for them looking the other way - or working for them. >> since the student were taken, the municipal police have been removed from this area and now what we're seeing is that it's the federal police and the army in control of security here the problem is - that policy has been tried before and hasn't stopped the violence. in 2006, former president felipe calderon flooded mexico's streets with thousands of federal forces like the army and marines . ..and even now under the current president, enrique pena nieto,

kidnappings have reached record levels in some regions. like the state of tamaulipas, just across the us border with texas we've come to the town of nuevo laredo. it's the headquarters of one of mexico's most violent cartels - the zetas we're here to meet raymundo ramos, he runs one of the few human rights groups remaining in tamaulipas. in most of the cases of disappearances that have been brought to raymundo, families believe it's not cartels that are responsible...but federal security forces.

oralia told us the last time she saw her husband jose was here in their home around 1am then, she says, the marines told her and jose that they were mistaken and

left. but a few minutes later, they came back. when she went outside, she says neighbors told her that they'd seen the marines put jose in their vehicle and drive off. so she got in her car and followed the convoy and finally she arrived here, a motel in downtown nuevo laredo this is footage shot by local journalists that night - oralia can be seen with her children as well as relatives and friends of other men that had been detained during the same operation masked men in uniforms that say 'marina' navy, in spanish stand watch outside >> oralia's son told her that he

saw his father in the window - but then the curtain was drawn. in the aftermath, the navy would change their story about the events of that night several times first they said they had no contact with the men. then later, they said they had questioned them - and found them innocent - and that they dropped them off in a different town 2 hours away. jose and the other men detained that night were never seen or heard from again.

we'd been told that there are some people who've been released after being held by security forces - but are too scared to speak about it. but we got lucky, we found someone who was willing to tell us their story. so we had to change cars twice to be able to get here. we're going to

talk to a woman who was allegedly kidnapped by the police and taken to a detention center that was run by the police. so this is very important - because it shows in a way the complicity between security forces and criminal organizations. and how difficult the corruption situation is in this country she agreed to speak with us if we protected her identity. two months before we met her, she was with her son in the car, driving home after work when, she says, they were stopped by armed men.

she told us she and her son were beaten repeatedly and moved to different locations through the night and into the day - until their family realized what happened and was able to have them released. she says a few weeks after they were released, her son was called out onto the street by a group of men. when he left the house, he was gunned down.

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>> in the case of the students, while there have been some allegations that federal forces could have been involved in the attack, there's been no hard evidence. but there is the question -

could they have stopped what happened that night? this is where three students were killed on september 26th and 43 others taken by the municipal police and handed over to criminal organizations. the military battalion is about a mile away from here25 but they say that they didnõt hear anything that night. this one shot that was fired that night whether or not they heard gunfire, the army never intervened. and since then, offials have said that the battalion never received an order to respond we repeatedly asked to speak with someone from the defense ministry, but got no response. so we went to talk to jose francisco gallardo, a former army general, to try to get insight into what may have happened.

omar says the only thing they wanted from the army was help for their injured friends.

despite there being a large military presence in guerrero as part of the government's operations against cartels, even before the students were attacked, criminal groups seemed to have no problem taking control of places like iguala... ...receiving regular payments from mayor jose luis abarca, and taking control of the local

police, going so far as to choose new recruits, according to the government. >> what worries me the most is that you have a complete vacuum of state institutions that are supposed to investigate, to prosecute, to prevent such level of forced disappearances like is happening in mexico. >> edgardo buscaglia is an investigator who has worked with numerous governments and civil society to combat corruption, particularly in mexico. >> pena nieto made a lot of noise about fighting corruption but you don't see any actions implemented yet. a perfect storm is taking place where the cost of kidnapping a human being and killing them is almost zero.

>> our requests to speak with the president and the attorney general were denied. but at the beginning of our time in mexico, before the students were kidnapped, we did have a chance to speak to the country's interior minister... miguel osorio chong is one of pena nieto's closest advisors.

>> two months later, the 43 students from ayotzinapa were taken and disappeared.

juana and her family have one final step in the forensic process ... their dna will be compared to the remains they were given to find out if it is their daughter, damaris...or someone else.

>> 5 months after the new test, the family received the results from franco the remains the government gave them belonged to their daughter - damaris.

as the government tried to close the case of the missing students, the parents and surviving

students traveled to mexico city to press their case at the senate and even as president pena nieto told the country it was time to move on... the protests have continued, with the parents at the frontline, trying to make sure their sons are not forgotten ...in a war where it becomes increasingly unclear whose side the government is on.

four months after the students disappeared, the country's attorney general - without significant new evidence or forensic confirmation - declared them dead - essentially closing the case.

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