On March 5, 16-year-old Amy Yu went missing. She was last seen standing at a bus stop with her younger brother John.

As Dearly previously reported, her mom initially reported her as a runaway because she believed that Yu had disappeared with a 45-year-old man named Kevin Esterly. According to the Allentown, Pennsylvania Police, Yu and Esterly are said to be in a "secret relationship."

Yu and Esterly first met at the church both of their families attend, the Allentown Morning Call reports. According to WFMZ-TV, the pair became close when the teen befriended Esterly's oldest daughter, traveling on multiple family vacations with the Esterly family.

Yu's mom told authorities that she found text messages on her daughter's phone that made it seem as though Esterly was her "boyfriend." However, it appears as though Yu's mom was the only one aware of the red flags.

WFMZ-TV spoke with individuals who know Yu and Esterly well. They said no one noticed anything weird between the teen and the father and are "shocked" by what is happening.

Prior to Yu and Esterly's disappearance, Esterly had been recorded signing the teen out of school a total of 10 times between November 2017 and February 2018.

Her mother found out about it when she went to pick her daughter up from school one day and the office told her that she had already been picked up by Esterly.

Knowing that Esterly wasn't one of the people listed as an emergency contact, the mother questioned how the school could allow her daughter to go with a stranger 10 different times.

As WFMZ-TV reports, further investigation into the matter revealed that the teen had changed her own emergency contact list, claiming Esterly to be her "stepfather."

Police were notified that Esterly had signed the 16-year-old out of school without her mother's permission on February 9, and were also called to the Esterly's home on February 10.

According to WFMZ-TV, Esterly and Stacey were involved in a fight about Esterly refusing to let his wife see his Facebook page. They were both charged with harassment. It is currently unclear how much Esterly's wife, Stacey, knew about the alleged relationship.

Yu and Esterly have now been unaccounted for for more than a week. Yu's mother told CBS This Morning that she is praying for her daughter's safe return.

The Department of Homeland Security is now involved in the case.

Both Yu and Esterly took personal documents with them when they disappeared. Esterly also withdrew $4,000 from his wife's account.

A warrant is now out for Esterly's arrest for interference with the custody of a child.