EDITOR'S NOTE: Amy Yu and one of the girls reported missing have been found by authorities..
Authorities say the girls pose no threat to the public.
Allentown Assistant Chief Gail Struss says his department was advised of Yu's disappearance in case she tries to return to the city.
Federal agents found Yu and 45-year-old Kevin Esterly in Playa del Carmen in March. He remains in Lehigh County Jail in lieu of $500,000 bail on charges including child custody interference and corruption of minors.
The two went missing Feb. 9, shortly after authorities were notified the pair may have been involved in a romantic relationship. In March, Yu's mother told CBS News' Meg Oliver that Esterlyat church.
In an interview with Dr. Phil in May, Esterly's then-wife Stacey said she didn't believe their relationship was sexual.
"In my heart, I don't want to believe they were sexual and I'm hoping that this was just an infatuation with him trying to help her as a father figure and he got sucked into something," Stacey told Dr. Phil of her husband's relationship with Yu.