The hepatitis C outbreak at Exeter Hospital is being investigated by the state Attorney General's Office after state health officials determined a hospital employee is likely to blame for infecting at least 20 patients.

Prosecutors have been working with state police and the Exeter Police Department, “to determine what, if any, criminal laws have been violated in regard to the outbreak,” Attorney General Michael Delaney said in a statement.

The outbreak — linked to an employee who used syringes to inject drugs — has set off what is expected to be a flurry of civil litigation.

A 61-year-old Newmarket woman filed a lawsuit Thursday against the hospital in Rockingham County Superior Court, claiming she was infected with the potentially fatal liver disease after being treated in February at Exeter's Cardiac Catheterization unit.

She learned about a week ago from the hospital and state officials that she was among those infected, attorney Michael Rainboth of Portsmouth said.

“It's not often where you see a medical provider that does something so outrageous,” Rainboth said. “Obviously this is a tragedy for everyone involved. It's not something you expect to see in New Hampshire.”

Rainboth said the hospital should have had procedures in place — such as tight inventory controls of drug doses and needles — to guard against patients being infected.

Manchester lawyer Peter McGrath, a former federal prosecutor, said Thursday he intends to file a class-action lawsuit against the hospital.

However, such a lawsuit would have to go through a multi-step approval process.

Kevin Callahan, president and CEO of Exeter Health Resources, apologized to patients during an interview with WMUR-TV on Thursday and pledged that if the hospital is responsible for the outbreak, “we will be responsible for that cost.”

The hospital reported the first case to the state's Department of Health and Human Services in mid-May.

State health officials have already tested hundreds of samples to determine the scope of the outbreak — of the 731 samples tested as of Wednesday, 629 were determined to be negative.

All of the cases so far were patients treated at Exeter's Cardiac Catheterization Lab and its recovery unit, but investigators are now looking into whether the employee worked in other parts of the hospital, health officials said.

State health officials have expanded their probe in the last week to test patients as far back as Oct. 1, 2010.

The state Department of Health and Human Services announced it would host a public forum at Exeter High School auditorium, 315 Epping Road, Friday at 6 p.m.

“We realize there is a great deal of anxiety and that many people have questions,” Dr. Jose Montero, director of public health, said. “And as we have done for other outbreaks, we want people to be able to hear firsthand how the investigation has evolved and where we are in the process.”

State health officials are working with the federal Centers for Disease Control in their investigation.