. >> reporter: 43 years later, what happened to brenda davidson is still a mystery. rumor has it she snuck out of school and caught a bus to run away. there's never been proof she bought a ticket lisa says her parents felt helpless and grew frustrated with the investigation. one phone call between her mother and a detective still stings today. >> he said that they ruled her as a runaway and my mom was an unfit mother. >> one of the cases here -- >> reporter: in alexandria, virginia, it's a familiar story for callahan walsh. a child advocate with the national center more missing and exploited children or nicnac. >> parents were quick to realize there was little help. >> reporter: his parents started the center in 1984 after his brother adam, a brother he never knew, was kidnapped from a florida shopping mall and brutally murdered. shocked to learn there was no national crime system for missing kids. the walshs helped pass the missing children's assistant act of 1984 which led to the creation of nicnac. >> i always grew up with parents telling we need to make sure adam

. >> reporter: 43 years later what happened to brenda davidson is still a mystery. rumor has it she snuck out of school and caught a bus to run away. there was never proof she bought a ticket or boarded a bus. lisa said her parents grew frustrated with the investigation. >> they said they ruled her as run away and my mom was an unfit mother. >> reporter: in alexandria, virginia it is a familiar story for walsh, a child advocate for center for missing children. his parents started the brother he never knew was kidnapped from a florida shopping mall and brutally murdered. the walshs helped pass the missing children act of 1984 which led to the creation. >> i always grew up with my parents telling me we need to make sure adam didn't die in vein. >> reporter: in the basement of this five story building you will find this room, a door with no marker. a place that a lot of people know about but rarely visit. this is the first time that any video cameras have been around in. hundreds of thousands of files each a different case, each of them representing a child that has be