ST. LOUIS • The Department of Veterans Affairs is warning hundreds of veterans that they may have been exposed to viruses from dental work performed at the St. Louis VA Medical Center.

The federal agency said it was mailing letters to 1,812 veterans treated during a 13-month period ending in March at the clinic at its John Cochran hospital. The letters say the risk of infection is low but offer free blood testing to screen for the hepatitis B virus, the hepatitis C virus and for HIV (human immunodeficiency virus) infection.

The VA letter said that dental equipment "may not have been cleaned correctly."

"We deeply regret that this situation occurred and we assure you that we are taking all the necessary steps to make certain that testing is offered quickly and results communicated timely," the letter reads.

The letter adds, "We want you to know that the staff at the St. Louis VA Medical Center is doing everything possible to address this situation and prevent it from occurring again."

VA spokeswoman Laurie Tranter said Tuesday night that the problem was uncovered during an inspection that took place March 9-12. Dental services were suspended on March 11 and resumed on March 26. She said the problem was related to "proper processing of dental equipment."

A statement released by the federal agency said that an internal VA Clinical Risk Assessment Advisory Board had determined that the risk of infection was "extremely low" but had nonetheless decided to contact veterans and offer testing.

As to why it took more than three months for veterans to be told that they might have been exposed to HIV and hepatitis viruses, Marcena Gunter, spokeswoman for the VA in St. Louis, said the agency needed time to figure out the risk level from the potential exposure.

"We are very conservative in our actions," Gunter said.

Rep. Russ Carnahan, D-St. Louis, said Tuesday night that he had requested that the federal agency conduct a formal investigation. In a letter Tuesday to VA Secretary Eric Shinseki, Carnahan called the potential exposure an "indefensible breach of standard operating procedure."

Carnahan added in an interview that he had requested a congressional hearing.

"I'm mad as hell about this. This is completely unacceptable," he said. "We have got to understand very quickly what happened, get the people who were exposed in for testing and make sure that this doesn't happen again."

Thirty veterans were tested at Cochran on Tuesday. There was a range of reactions, from calm to one person who shouted, "I take care of my grandchildren, what's going to happen to them?" said Jennifer Cunningham, a nurse who was briefing the veterans on their exposure risk.

Veronica Lynn Williams, 57, of Swansea, Ill., had a tooth reconstructed at the medical center and got one of the certified letters about the safety lapse at Cochran. She immediately set up an appointment. "It's not something you want to let linger," she said Tuesday at the medical center.

But Williams said she wasn't angry. "Things happen," she said. "I don't fault the VA. I have been treated here for several years, and my life has been saved on several occasions."

Cochran's procedures have come under scrutiny before. In 2003, an independent agency that evaluates and accredits most of the nation's hospitals took the rare step of placing the St. Louis VA Medical Center on "conditional accreditation" after a surprise inspection uncovered problems at Cochran. The hospital regained full accreditation by the end of the year.

In 2007, after reports about poor treatment and rundown conditions at Walter Reed Army Medical Center in Washington, officials at the VA's St. Louis area facilities said they were working to solve similar problems.

Veterans, their families and organizations that represented them cited long emergency room waits, poor nursing care, difficulty scheduling appointments and delays in getting needed services. They also complained about outdated buildings, indifferent employees and paperwork nightmares.

Hospital officials said at the time that a bigger budget, additional hiring, millions of dollars in planned and completed renovations and customer service training were aimed at addressing many of the patients' concerns.

Sterilization of equipment also has been a problem before.

Hospital leaders closed the supply processing department for two weeks in December and January to train staff and to sterilize all endoscopes, which are used in colonoscopies and other procedures.

A month later, after receiving a complaint about endoscope sterilization, Veterans Affairs inspectors visited the hospital and found several health and safety infractions. The temperature in the sterilization area was too high, rags and gloves were 'strewn about" in the decontamination areas, filters had not been changed as required, a technician was not wearing protective gear, chemical test strips were left exposed, emergency exits were blocked and employees were unsure whether an unattended endoscope was sterile, according to an inspection report issued in April by the VA's Office of Inspector General.

Michael Shea, 58, a Cochran patient who was treated for an abscess on his arm, was critical of the hospital's latest safety lapse, although he hasn't been to its dental clinic in decades.

"That would make one think twice about going to the dental department," he said, taking a smoke break out front Tuesday. "Somebody wasn't doing their job. The hospital should be a clean place."

But Shea, an Army veteran, said he'd continue to come to the VA for medical treatment. "I don't really have other options," he said. "I come because I don't have health insurance."