SALEM — A woman who worked at a Methuen medical company has been charged with stealing prescriptions from her employer, then forging and passing them at two Salem pharmacies.

Kimberly Gynan, 44, of Derry turned herself in to police Thursday on four felony counts stemming from allegedly writing herself fake prescriptions over a five-month period earlier this year.

Gynan is alleged to have stolen the prescriptions from her employer, Branch Medical Group in Methuen, forged and passed 22 of them for hydrocodone and oxycodone at Rite Aid and Walmart pharmacies, both in Salem, police Capt. Shawn Patten said.

The Massachusetts State Police drug task force began investigating the stolen prescription forms earlier this year. When it was determined the prescriptions were being passed in Salem, local police joined the investigation, Patten said.

Police reports have been sent to the county attorney's office, and more charges could be brought against Gynan, Patten said.

"It was a lengthy, detailed investigation, and we were able to bring forward four initial felony charges," Patten said. "Overall, this case will ultimately be handled by the Rockingham County attorney's office."

Gynan could potentially face up to 40 more charges in New Hampshire, and could face charges from Massachusetts, Patten said.

The case remains under investigation.

Gynan was released on $500 cash bail pending arraignment.

Patten said Gynan told police she needed the medication for back pain.