nisa's parents, robert mickens and liz alvarado, were desperate. alvarado: we were looking so hard, and we could not find her. lisa: after a police van drove past their house, they followed it. robert mickens: we came across another officer a few blocks away, we're looking for our daughter, and then he's like, "do you have a picture?" i was like "yeah, i have a picture." i showed him the picture and he looks at me, looks at liz, and goes, you know, "i'm sorry to tell you, but that's your daughter." lisa: the students were both murdered by ms-13 members, who used machetes and bats. since january 2016, the gang has been responsible for 21 deaths -- 22 deaths on long island, inspiring a visit from president trump earlier this summer. pres. trump: they have transformed peaceful parks and beautiful quiet neighborhoods into bloodstained killing fields. they're animals. >> the purpose of the gang is take control of the town. take control of everything that's going on with drugs, trafficking, prostitution. lisa: this man spent seven years in ms-13 before h