Federal authorities say a former surgery technician may have exposed thousands of patients to hepatitis C when she swapped her own dirty syringes for ones filled with a powerful narcotic.

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

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

Rose Medical Center said Parker was fired before the hepatitis cases were discovered.

An affidavit says at least nine surgery patients at Rose have tested positive for the hepatitis C, which is incurable. About 6,000 patients are being advised they may have been exposed and need to be tested.

Rose Medical Center said it was contacting 4,700 patients who were treated in the main hospital or Wolf Building between October 21, 2008 and April 13, 2009.

"This is a personal issue for all of us," Rose Medical Center said in a written statement Friday. "We value every patient who comes here for care. It is impossible to adequately express how deeply sorry and angry we are that the unconscionable acts of this terminated employee may have put some of our patients at risk."

Rose has established a Patient Care Line to answer questions from patients and their families. The line will be open from 7 a.m. to 7 p.m. Mountain Time. The number is 303-329-7500.