A mother's worst fears: How Amy Yu disappeared with 45-year-old man

Police say Amy Yu and her "secret boyfriend," Kevin Esterly, covertly ran away to the resort town of Playa del Carmen. Photo / AP

In the weeks since Amy Yu disappeared, the 16-year-old's family has replayed eight years of warning signs about the intensifying relationship between the high school student and the 45-year-old father of four with whom she fled to Mexico.

Police say the girl and her "secret boyfriend," Kevin Esterly, covertly ran away to the resort town of Playa del Carmen together this month. An Amber alert and an international search ensued, and the pair returned to the United States under vastly different circumstances on Saturday.

Amy was reunited with her family. Esterly remains in federal custody, awaiting extradition and charged with felony interference of the custody of a minor.

Esterly first met Amy when she was 8. He was a father of four in his late 30s who was introduced to Amy's family at their house of worship near Allentown, Pennsylvania, according to the Allentown Morning Call. Over the next few years, Amy accompanied Esterly's family on several beach and camping trips, becoming close to his oldest daughter - and, apparently, to him.

There was a secret, hidden aspect to the relationship between the teen and the middle-aged man.

Esterly had signed the girl out of school 10 times since November without her family's permission, acts made easier by the fact that she had listed him as her stepfather on emergency-contact forms, according to CNN. The disappearing acts got so bad that, at one point, Esterly was prohibited from being on the grounds of Lehigh Valley Academy Regional Charter School.

Amy's mother, Miu Luu, confronted Esterly about the pickups, the Morning Call reported, but he called them "lies and speculation."

But worse was happening in secret, family members and authorities believe. The girl had exchanged hundreds of text messages with the man.

As details about the relationship trickled out, there were fights over phones that contained a troubling electronic trail. Amy's mother told CNN that texts she discovered on her daughter's phone made the teen and Esterly seem like girlfriend and boyfriend. Esterly and his wife had gotten into a scuffle as she tried to look at Facebook posts on a phone, and police were called.

The pair ran away together to Mexico resort town Playa del Carmen. Photo / Supplied

And a few blocks from Amy's home, police told the Morning Call, Esterly had rented out an apartment that he kept secret from his family.

As more details came out, tensions escalated.

Last month, Esterly and his wife came to Amy's house and got into a shouting match with the teen's mother that was so vitriolic that police were called.

"They were yelling about they think that Amy and Kevin are sleeping together and doing stupid stuff," Amy's brother, John, told CNN.

Esterly denied the affair, the Morning Call reported. Police told the Esterlys to leave the Yus' home and never come back.

But during that time, investigators believe, Amy and Esterly were making plans to leave Allentown together.

On March 5, Amy's mother called police and said her daughter had disappeared.

The woman told authorities that she had dropped her daughter off at the bus stop that morning but that Amy never arrived at school.

She made other troubling discoveries at home: The girl's passport and other personal documents were gone. So was some of her mother's jewelry.

Esterly had also vanished, along with $4,000 he had withdrawn from his wife's bank account. His family reported him missing two days after Amy's family filed their missing-person report.

"They know or planned to do this,"John Yu told CNN. "I think it was both of their ideas because Amy is not that stupid."

On March 7, the Allentown Police Department issued an Amber Alert. The 4-foot-11 girl was missing and believed to be with Esterly. They "could possibly be traveling together in a 1999 red Honda Accord" with a Pennsylvania license plate. The girl, the alert said "could possibly be endangered."

But the car was apparently only the first part of their plan.

A little later, Frank Castrovinci, a man on a flight headed to the Mexican resort town of Cancun, saw a teenage girl and a middle-aged man board the plane just before the doors were secured.

They had backpacks or duffel bags, Castrovinci told CNN. He said he assumed it was "a guy who adopted a girl and she looked obviously much younger," but the pair's behaviour after they found their seats made him change his mind about the nature of the relationship.

"It seemed odd the way she was leaning up against him and how he was rubbing her leg and it seemed like his hand was quivering a little bit," Castrovinci said.

At home a few days later, he turned on the TV news and saw a report about a missing girl and a middle-aged man and recognised the faces on his screen.