A 25-year-old dental assistant is behind bars Thursday, accused of sexually assaulting two young girls during dental treatments.

Police said it's a difficult case because the alleged victims are so young, just five and six years old.

Investigators believe Tommy Medina took advantage of the girls during their visits to OC Kids Dental in Santa Ana, where he worked for the last three years.

The dental office is specifically geared toward children. That is why Madahi Jimenez told NBC4 she brings her children to the office. There are toys if they behave. The clinic takes her insurance.

Her daughter, Alexa, was hoping to have a tooth pulled Thursday, but the office was closed.

Jimenez said that what she remembers about Medina is that he would treat patients in a room away from their parents.

“He would ask every parent to go outside … like, to wait for them in the waiting room,” she said.

Police said that's when the sexual assaults took place.

“Often 5- and 6-year-old kids have no idea of the gravity of what's occurred during an assault like this and don't know how to express themselves,” said Cpl. Anthony Bertagna, spokesman for the Santa Ana Police Department.

Charges were never filed in the first reported case. Then, police said, they got another complaint from a young patient last year.

They believe given Medina's tenure at the clinic, there may be other victims.

Authorities could not immediately explain why Medina was still working in the dental office after he'd been investigated for the first alleged attack.