An employee misusing drugs is the most likely cause of an outbreak of hepatitis C among patients who were treated at the Exeter Hospital’s cardiac catheterization lab, New Hampshire’s public health director said Wednesday.

‘‘Based on all the testing we've done, based on all the interviews we've performed with employees and with patients, the review of the hospital data — all of this information points toward drug diversion as the most plausible explanation,’’ Dr. Jose Montero said.

A total of 20 people, including a hospital worker, have been diagnosed with the same strain of the liver-destroying virus since the state began investigating the outbreak last month. Montero would not comment on the specific employee suspected of causing the outbreak or say whether law enforcement is involved, but he said drug diversion generally involves someone using a syringe to inject themselves with medication meant for patients and then re-using the syringe on patients.

‘‘This is really disturbing. We as a department want to make sure health care quality is maintained across the board,’’ he said. ‘‘Certainly this is a really unfortunate situation. Nobody goes to a health care facility expecting to get sick.’’

Hepatitis C is a viral infection transmitted by blood. It causes inflammation of the liver that can lead to chronic health issues.

The Exeter investigation began in mid-May when four patients were diagnosed with an identical strain of the virus, and the only link officials could find was that all had been treated at the lab. Officials initially asked anyone treated at the lab since August to get tested; on Wednesday, Montero said that request has been expanded to include all of the lab’s patients since October 2010.

The lab was closed for a week in late May but was allowed to re-open after authorities determined there was no evidence that disposable equipment was being misused, that no permanent equipment was contaminated and that there was no further risk of transmission via lab employees, Montero said.

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, and 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 were found to have hepatitis C after being exposed.

In New Hampshire, Montero said about 730 people have been tested so far, and several hundred more are expected. The state had been notifying those who did not test positive by mail but is now calling them, recognizing that patients are anxious to learn the results.