DENVER — A former surgery technician may have exposed thousands of Colorado patients to hepatitis C when she swapped her own dirty syringes for ones filled with a powerful narcotic, federal authorities said Thursday.

Kristen Diane Parker faces criminal charges for allegedly making the swaps while working at Audubon Ambulatory Surgery Center in Colorado Springs and Rose Medical Center in Denver.

Authorities say Parker admitted to changing out syringes containing a saline solution with ones filled with the painkiller Fentanyl. Parker injected herself with the drug, according to a complaint filed Thursday in U.S. District Court in Denver.

An affidavit by Mary F. LaFrance, an investigator for the U.S. Food and Drug Administration, says at least nine surgery patients at Rose have tested positive for hepatitis C, which is incurable. About 6,000 patients are being advised they may have been exposed and need to be tested.

Hepatitis C is a blood-borne disease that can cause serious liver problems, including cirrhosis or liver cancer. The illness is treatable, but there is no cure. Symptoms can include nausea, diarrhea, fatigue, pain and jaundice.

Rose Medical Center officials told a news conference Thursday night they were working with the Colorado Department of Public Health and Environment to determine whether Parker was the source of the virus.

It could not be determined Thursday night whether Parker had an attorney.

Parker worked at Rose from Oct. 21, 2008, until April. Hospital officials say she was suspended April 13, before they learned of the cases, and then fired. She had failed a drug test by testing positive for Fentanyl.

Parker went to work for the Audubon surgery center shortly after being fired. She worked there from May 4 until Monday, Dr. J. Michael Hall, Audubon's medical director, told The Gazette in Colorado Springs.

If convicted of tampering with a consumer product and other charges, she faces up to 10 years in prison and a maximum $250,000 fine on the most serious charge of tampering.