SAUSALITO, CA (ASRN.ORG) -- Edgar Barnes was relieved, almost buoyant, on Friday for the first time in many weeks. He had just emerged from a state-sponsored health clinic here where he was tested for hepatitis C. His test came back negative.

I’m excited,” said Mr. Barnes, 74, of nearby Raymond, allowing himself a big smile. He said that he and his wife would celebrate by going out for seafood.

Mr. Barnes is one of thousands of people in New Hampshire who have been anxious that they might have hepatitis C, a chronic disease that can lead to cancer and is a major cause of liver transplants.

They were all patients at Exeter Hospital, not far from here, between April 2011 and May 2012, when a medical technician on the staff was linked to a hepatitis C outbreak. The outbreak is one of the largest in recent history and is complicated because the technician, David Kwiatkowski, 33 — who may have had the disease since at least June 2010 — had worked at 18 hospitals in seven other states (Arizona, Georgia, Kansas, Maryland, Michigan, New York and Pennsylvania) over the last decade. He was fired from at least two hospitals but was hired subsequently by four others.

Several federal and state agencies across the country are conducting a widespread investigation to find out when Mr. Kwiatkowski was first infected, how exactly he might have transmitted the disease to others and to whom he might have transmitted it; testing is under way in some of the other states where he worked.

“Its reach is unprecedented, and we’re the tip of the spear in the investigation,” John P. Kacavas, the United States attorney for New Hampshire, said in an interview. “In terms of volume, scope and intensity of work involved, it’s a mammoth effort.”

Law enforcement officials said that Mr. Kwiatkowski engaged in a classic bit of “drug diversion.” In such cases, a hospital employee steals a fresh syringe, injects himself with it — thus almost certainly contaminating it — refills it with some other liquid and slips it back into place, whereupon an unsuspecting doctor or nurse uses it on a patient.

A new report this week by the federal Centers for Medicare and Medicaid Services said that at Exeter syringes were left unattended on medication carts by nurses in the cardiac catheterization lab, where Mr. Kwiatkowski mainly worked.

Hospital officials said they have since made sure that such drugs are kept secure until they are needed.

So far, 31 of the more than 1,200 people in New Hampshire who were patients in the cardiac catheterization lab have tested positive for Mr. Kwiatkowski’s same strain of hepatitis C. (About 14 others also tested positive, but they had a different strain.) The state health department on Friday began testing the first batch of 3,300 more people who had been patients in other parts of the hospital where Mr. Kwiatkowski had also worked.

State health officials said they would not discuss how many people tested positive on Friday because they would not know for at least a week if those people were carrying the same strain as Mr. Kwiatkowski and would be considered part of the outbreak. About 1.5 percent of the population has hepatitis C, although they might not know it, said Dr. Jodie Dionne-Odom, the deputy state epidemiologist.

The last big outbreak of hepatitis C occurred in Texas two decades ago, when a surgical technician infected about 40 patients, according to the Centers for Disease Control and Prevention.

Mr. Kwiatkowski is being held at the Strafford County House of Corrections in Dover, N.H., on two federal charges — tampering with a consumer product and stealing drugs, primarily fentanyl, a powerful anesthetic that is about 80 times more potent than morphine.

Mr. Kacavas, the United States attorney, had expected to seek an indictment against Mr. Kwiatkowski this month but because of the complexity of the case he postponed any action until early October.

Mr. Kwiatkowski has said he is “not a shooter,” according to an affidavit filed last month in federal court by an F.B.I. investigator. Mr. Kwiatkowski has also said that he contracted hepatitis C only recently. His public defender, Bjorn Lange, declined to comment.

The affidavit said there were indications while Mr. Kwiatkowski was working at Exeter that he may have been abusing drugs. Hospital employees told investigators that he had “fresh track marks” on his arms and that he sometimes sweated profusely, vomited, appeared shaky, had bloodshot eyes and a red face and once had “white foam around his mouth.” They said he frequently showed up at procedures to which he had not been assigned.

His parents, who live in Michigan, told authorities last month that he had Crohn’s disease and was taking several medications. While they were not aware that he might be abusing drugs, they said he did have “issues with alcohol, anger and depression.”

Mr. Kacavas said: “He was an itinerant medical technician with a virulent drug problem. How functional he was, I have no idea. But given the number of drug diversion cases of which we are aware, there are functional drug addicts out there.”

In response to the F.B.I. affidavit, the hospital said it had “no report that any employee suspected him of diverting medication from the hospital.” Dr. Thomas Wharton, medical director of Exeter’s cardiac catheterization lab, said in a statement that Mr. Kwiatkowski was “the ultimate con artist and an extremely good cardiac technologist who pulled the wool over everyone’s eyes.”

Mr. Kwiatkowski had been fired at least twice. In 2008, after 47 days on the job at UPMC Presbyterian in Pittsburgh, he was found inside an operating room where he had not been assigned. In 2010, after 11 days at Arizona Heart Hospital in Phoenix, he was found passed out in a bathroom with syringes and needles.

In both cases, there was limited reporting of the incidents, including to the American Registry of Radiologic Technologists, a private professional group. The group suspended Mr. Kwiatkowski last month, but only after he was arrested, telling CNN that it took no action earlier because it had no firsthand evidence against him.

Exeter officials have said they were “saddened” that the hospitals did not report Mr. Kwiatkowski’s past behavior to law enforcement officials.

“This inaction,” the hospital said in a statement on its Web site, “allegedly resulted in Kwiatkowski being able to secure employment in other hospitals around the country, including Exeter Hospital, resulting in this hepatitis C outbreak that has touched thousands of individuals across the New Hampshire seacoast and beyond.”

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