CONCORD, N.H. — Federal authorities have joined the investigation into the hepatitis C outbreak at Exeter Hospital.

U.S. Attorney John Kacavas said Tuesday his office has brought in personnel from the Food and Drug Administration and FBI to determine whether the outbreak at the hospital’s cardiac catheterization lab is a criminal or civil matter. Doing so isn’t uncommon in such situations, he said.

“This case presents a matter of public health and safety, and we have statutory authority at the federal level that fit these precise circumstances,” he said.

A hospital worker and 19 patients at its cardiac catheterization lab have tested positive for the liver-destroying disease since the investigation began last month. State health officials suspect a lab employee’s misuse of drugs led to the outbreak. They have not provided any details about the suspected individual, but have said such cases generally involve workers stealing medication meant for patients, injecting it into themselves and then reusing the syringes on patients.

The state attorney general’s office also is investigating the outbreak, and a Concord lawyer is preparing a class action lawsuit against the hospital. Attorney Peter McGrath said Tuesday that 23 people have joined his suit, which accuses the hospital of negligence in supervising its staff. The suit seeks damages for patients who were infected with hepatitis C as well as those who paid for medication they did not receive.

The hospital, which has declined to comment on the lawsuit or criminal investigation, has asked anyone who was treated at the hospital since October 2010 to get tested for hepatitis C, a viral infection transmitted by blood that causes inflammation of the liver and can lead to chronic health issues.

It’s unclear how ill any of the 20 infected people have become; the investigation began with four people who were diagnosed with the same strain of the virus around the same time and authorities determined that the hospital lab was the only common link among them.

More than 800 people have been tested so far.

Starting today, those who have yet to be tested will have two new locations where testing will be done. In response to complaints from people who did not want to return to the hospital for testing, testing sites were set up at two locations, both affiliated with Portsmouth Regional Hospital. One is in Portsmouth, the other in Hampton.

State and local health departments aren’t required to report such outbreaks to the Centers for Disease Control and Prevention, but the agency was notified of 13 outbreaks nationwide between 2008 and 2011. Of those, seven occurred in outpatient facilities; most were traced to unsafe injection practices.

At least two have resulted in criminal charges, including a Colorado woman who was convicted of stealing syringes filled with painkillers from two hospitals where she worked and replacing them with used syringes. The syringes were later used on surgical patients, and up to three dozen patients were found to have hepatitis C after being exposed.