Missing girl, 16, may have flown to Cancun with 45-year-old man: Police Kevin Esterly, 45, and Amy Yu, 16, have been missing since March 5.

A missing 16-year-old girl from Pennsylvania is believed to have flown to Cancun, Mexico, with a 45-year-old man around the time her mother was reporting her missing, police said.

Kevin Esterly and Amy Yu, 16, have been missing since March 5, and police believe the teen left with the man willingly.

When Yu did not return home from school that night, her mother reported her missing, the Allentown Police Department said today.

Two days later, police received information indicating that the night of March 5 Yu and Esterly took one-way flights from the Philadelphia International airport to Cancun, connecting through the Dallas/Fort Worth International Airport, police said.

An Amber Alert has been issued in Mexico for Yu.

"Prior to the Amber Alert in Mexico, the information that Amy Yu and Kevin Esterly were likely in Mexico was not released as it is believed that Kevin Esterly and/or Amy Yu are monitoring media reports," police said in a news release today.

Yu's mother, brother, friends and classmates miss her and want her to return home safely, police said today.

"Amy, if you are uncertain how to come home or who to contact for help, you may simply reach out to a law enforcement official, a resort staff member, or contact your mom via text message, email or social media, and we will work to reunite you with your family right away," police said.

Esterly, who is married, met the teen at church, and Yu appears to have been friends with one of Esterly's daughters, said Gary Hammer of the Colonial Regional Police, which has jurisdiction over Yu's school, Lehigh Valley Academy.

Philadelphia ABC affiliate WPVI spoke to John Yu, Amy's brother, who said he knew Esterly from church.

"Yeah I know him, he was a family friend of mine and we met him at church and pretty much that was about it," John Yu said.

When Yu was asked by members of the Leigh Country Child Advocacy Center whether she was having a relationship with Esterly, she denied it, Hammer said.

However, Yu altered her school records and listed Esterly as her stepfather, Hammer added. And at least 10 times between December and Feb. 9, Esterly signed her out of school early, he said.

Feb. 9 was the final day because that's when Yu's mother came to the school to pick up her daughter, "and the school said her stepfather already signed her out of school," Hammer told ABC News last week.

"The mom explained she is a single mother," Hammer said. "There is no stepfather."

Lehigh Valley Academy confirmed that Esterly has been on school grounds before and was last there on Feb. 9.

"After that date, due to circumstances we cannot disclose pursuant to student privacy constraints, he was prohibited from entering school grounds, and the police were to be notified if he returned," the school said in a statement.

The school said in its statement, "Due to federal and state privacy constraints, the school is prohibited from releasing any additional information about the student or the facts and circumstances surrounding the situation unless the parent provides express consent for the school to do so.

"We are, however, working closely with the Colonial Regional Police and the Allentown Police and are providing whatever information and assistance we can to assist them and Amy’s family during this difficult time," the school added.

The school called the Colonial Regional Police immediately and it started investigating. The department found video of Esterly signing the teen out and leaving with her, Hammer said.

On March 7, a family member of Esterly reported him as missing/endangered, and that same day, an arrest warrant for Esterly was obtained charging him with interference with the custody of children, the police said.

Yu is about 4 feet 11 inches tall and 90 pounds, while Esterly is 5 feet 9 inches tall and 185 pounds, according to police.

Authorities believe they may be driving a 1999 red, two-door Honda Accord with the Pennsylvania plate KLT 0529.

Anyone who sees either of them is asked to call 911, local police or the Allentown Police at 610-437-7751.