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. 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 no vaccine for hepatitis C is available, according to the Centers For Disease Control and Prevention. Symptoms can include nausea, diarrhea, fatigue, pain and jaundice.

"It is impossible to adequately express how deeply sorry, and angry frankly, we are that the unconscionable acts of this terminated employee may have put some of our patients at risk," HealthONE president and Rose spokesman Jeff Dorsey said Thursday.

Dorsey added 4,700 letters are being sent out to patients who had surgery between Oct. 21, 2008, and April 13, 2009.

The letters warn the patients of the potential exposure to hepatitis C.

Only patients who had surgery in the Main Building or the Wolf Building should inquire about the free testing. People treated in the Emergency Room or women who had babies are not affected, doctors said Thursday.

While Rose officials did not disclose the former employee’s name, court documents showed Kristen Diane Parker, a former scrub tech at the medical center, is facing felony drug charges.

Court papers filed in Denver court and obtained by 7NEWS show Parker, 26 was arrested for allegedly filling syringes with saline and labeling them Fentanyl - which is used for pain.

“She would take the real syringe off the anesthesia cart and replace it with the syringe containing the saline solution,” an investigator wrote in a federal court document.

According to the document, “she would then inject herself.”

The court filing also said she believes she “diverted the syringes of Fentanyl on 15 or 20 occasions.”

Parker was interviewed by investigators June 30.

A MySpace page with her same name, age and occupation said, “I have a crazy fascination with needles.”

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.

A patient care line has been set up to answer questions. The number is 303-329-7500, and will be staffed from 7 a.m. to 7 p.m. Mountain Time.

Patients are asked to wait for the letter notifying them they may be affected before calling.

"We are casting as broad a net as we possibly can so not a single person could fall through the cracks,” said Dr. Don Lefkowits, a Rose physician.

He added, “It is not a highly transmissible disease, but is can be transmitted in that setting."

Dorsey reiterated the importance of patient safety and an internal investigation will help identify areas for increased prevention.

"The trust people bring to us is what we value the most," he said.

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 in Colorado Springs 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.

7NEWS reporter Lane Lyon contributed to this report.