Pentagon officials released a report Thursday detailing recurring problems at an Army bioterror facility that accidentally sent live anthrax to other labs for more than a decade.

At a press briefing in Washington, military officials said half the lots of anthrax produced at the Army's Dugway Proving Ground in Utah contained live anthrax after workers there failed to kill the bacteria before shipping it to other labs.

"By any measure, this was a massive institutional failure" involving a potentially dangerous biotoxin, said Deputy Defense Secretary Robert Work.

He presented the results of an investigation that was launched after the problem first surfaced in May. Officials said several problems contributed to the Dugway failures.

The Pentagon has ordered the Secretary of the Army to do an additional investigation that will specifically try to determine which individuals are to blame, Work said. Dugway officials should have recognized and corrected the problem, he said.

"We are shocked by these failures," he added. "It was absolutely inexcusable."

Dugways's new commander, Col. Sean Kirshner, declined comment, saying he needed time to review the report, said base spokeswoman Sheryl Grubb.

No illnesses have been reported, although 31 Americans - including eight who work for the military - have recently taken medication as a precaution.

Anthrax is most dangerous when it spreads through the air and gets into lungs. But the anthrax was shipped in liquid form in vials, officials said.

Dugway works with biological and chemical agents, and is the military facility that produces the largest amount of anthrax shipped to other labs for research.

Medical technicians are supposed to kill the anthrax bacteria with gamma rays, and then test samples from the lots to make sure the radiation succeeded in killing them. Investigators believe they were trying to kill too much at a time, and then doing inadequate testing afterward.

The problem came to light in May when a private commercial lab in Maryland tested a shipment from Dugway and found live bacteria.

Investigators from the Centers for Disease Control and Prevention and the Department of Defense found that 17 of 33 batches produced at Dugway since 2003 had live anthrax.

Anthrax from those batches was initially sent to 86 labs in the U.S. and seven countries. But some labs sent anthrax on to other facilities. So far, the CDC has traced samples to 183 labs, a CDC spokesman said Thursday.

Anthrax is a dangerous infectious disease caused by a bacterium found naturally in soil. It is rare, but people can get sick with anthrax if they come in contact with infected animals or contaminated animal products.