(Reuters) – A former hospital technician in New Hampshire who caused patients as old as 80 to become infected with hepatitis pleaded guilty to leaving dirty syringes for hospital use after he injected himself with stolen painkiller.

The technician, David Kwiatkowski, had previously admitted to knowing that he was infected with hepatitis C. In papers filed on Monday in federal court in New Hampshire, he pleaded guilty to 14 criminal counts related to seven cases in which he caused infections in patients ranging in age from about 40 to more than 80.

Kwiatkowski told police after his arrest in July 2012 that he realized his actions could “kill a lot of people,” according to court papers.

He was working at Exeter Hospital in New Hampshire in early 2012 when he began stealing syringes of the powerful pain medication Fentanyl. Hospital workers unwittingly used the refilled dirty needles on patients.

Kwiatkowski estimated he swapped out syringes on about 50 occasions while working at Exeter Hospital. He said he had also swapped out syringes at least 20 times when he worked in Kansas and 30 times in Georgia, court documents said.

He pleaded guilty to seven counts of tampering with a consumer product and seven counts of obtaining controlled substances by fraud. The charges carry a 30-to-40-year prison sentence.

After he began working at Exeter Hospital in 2011, hospital officials identified several unexplained cases of hepatitis C, and an investigation showed the outbreak came from drugs diverted by Kwiatkowski, who denied knowing that he had the disease.

Police searched his car and found an empty syringe with a blue “Fentanyl” label similar to those used at Exeter Hospital, along with several needles, one containing Kwiatkowski’s DNA.

After his arrest, he admitted to being diagnosed with hepatitis C in 2010 while working in Kansas and “swapping out” syringes of Fentanyl with those of saline solution at Exeter Hospital.

“It was all me,” he told investigators. “And I’m going to kill a lot of people out of this. … I’m killing a lot of people.”

Before moving to New Hampshire, Kwiatkowski worked at health care facilities in Kansas, Michigan, Maryland, Georgia, Pennsylvania, Arizona, Kansas and New York. He was twice fired and once resigned amid an investigation into missing controlled substances, according to court papers.

As a result, authorities conducted a massive investigation, recommending that more than 11,000 people get tested for possible hepatitis C infection.

The disease can liver damage, as well as other health issues, and can be fatal.

(Reporting by Scott Malone and Daniel Lovering; Editing by Leslie Adler)