Nearly two weeks after a 16-year-old Pennsylvania girl vanished with a 45-year-old man, the pair was found Saturday in Mexico and the man was arrested, authorities say.

Amy Yu was "unharmed and in good health," Allentown police said in a statement.

She and Kevin Esterly, the man police say she willingly ran away with, were escorted to Miami by US Homeland Security Investigations agents and the US Marshals Service.

Amy has been safely returned to her family in Pennsylvania, according to a statement from Immigration and Customs Enforcement (ICE) and Homeland Security investigators.

Esterly's arrest comes two days after Mexico's Attorney General issued an Amber Alert for Amy. The alert said Esterly was the suspect in her disappearance and that an investigation showed she was in Mexico with Esterly. Her well-being, the alert said, was at risk.

CNN has reached out to Mexico's attorney general and the federal police for comment, but has not heard back.

Esterly was expected to be extradited back to Pennsylvania.

The relationship

Mui Luu, Amy's mother, told CNN that Esterly knew the family from church, and that she's known Esterly since her daughter was 11. Esterly himself, she said, has four daughters and a wife.

Amy was a student at Lehigh Valley Academy Charter School, which confirmed to CNN that Esterly was last present at the school February 9. At some point after that, school CEO Susan Mauser said, the school prohibited him from school grounds. But Mauser said she couldn't elaborate, citing privacy concerns.

The police were to be notified if Esterly returned to the school, Mauser said.

Allentown Police Chief William Reinik said Esterly signed Amy out of school multiple times and that their relationship had been "going on for at least a year." Amy listed Esterly as her stepfather on her emergency contact form, according to CNN affiliate WFMZ. An affidavit of probable cause, obtained by WFMZ, states Esterly signed Amy out of school 10 times.

Police said Esterly and Amy exchanged hundreds of text messages, WFMZ reported. Amy's mother said the texts read as though Amy was texting her boyfriend.

Mauser said the school is working closely with both Allentown police and the Colonial Regional Police Department, which has jurisdiction of the area where the school is located.

The school is "providing whatever information and assistance we can to assist them and Amy's family during this difficult time," Mauser said.

It was also around early February that Esterly and his wife, Stacy, went to Amy's house. During the interaction, Amy's mother called police, who warned the Esterlys to stay away from their home.

"It was a meeting that time," John Yu, Amy's brother, told CNN. "My neighbors were there and my friends were there. My mom's friend was there. They were yelling about they think that Amy and Kevin are sleeping together and doing stupid stuff."

Kevin Esterly was angry, John Yu said, and denied his relationship with Amy. CNN's attempts to reach his wife, Stacy Esterly, have been unsuccessful.

On March 5, Amy's mother reported her as a runaway after she failed to return from school that day, Allentown police said. It was believed Amy boarded her school bus that morning, but police said they later found out she never got on her bus.

Two days later, Esterly was reported missing by a family member, police said.

Amy's mother said money, jewelry and her daughter's personal documents were missing, according to court documents, and Esterly's wife said $4,000 along with his personal documents were missing.

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

The age of consent in Pennsylvania is 16, according to the Pennsylvania Coalition Against Rape. Those who are at least 16 can legally consent to sexual activity, the group said, as long as the other person doesn't have authority over them.

The sighting

A passenger on a Cancun-bound flight told CNN he was "close to 100% certain" he saw Esterly and Amy boarding the plane with him at Philadelphia International Airport on March 5.

Frank Castrovinci said he saw a young Asian girl with "very long, straight hair" and a man in his 40s with glasses who boarded the plane late. They were asking people to switch seats so they could sit together. Each had a "backpack or duffle bag kind of thing."

"My initial thoughts were, here's a guy who adopted a girl and she looked obviously much younger," he said. "I thought it was interesting that the mother wasn't there."

It was during the flight that Castrovinci says he observed, through their body actions, that they weren't family.

"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," he said.

When Castrovinci returned home March 8, he said he looked at the local news and immediately recognized the young Asian girl and middle-aged man on TV as the same people he saw on the plane.