WASHINGTON, More than 800 radioactive sources for medical treatment, research, food irradiation and other uses -- potential ingredients for "dirty bombs" -- have been lost or stolen In the United States in the last five years, Reuters quoted the Nuclear Regulatory Commission as saying in a letter released on Friday.

The NRC said it had no reason to believe the material had been stolen to be made into radioactive bombs that could force mass evacuations and potentially cause deaths.

But critics said the missing material highlighted U.S. nuclear security failings.

"We've spent years trying to help the Russians control nuclear materials from the former Soviet Union, but it turns out that we have a 'loose nuke' problem right here at home," said Rep. Ed Markey, a Massachusetts Democrat who requested the NRC information.

The NRC licenses 2 million radioactive sources for a variety of medical, research and industrial uses. From 1996-2001, there were reports of 1,495 lost, stolen or abandoned sources, 835 of which were not recovered, the letter said.

"Some of this material could potentially be used for a so-called dirty bomb," an NRC spokesman said. "Most of these bombs would disperse radioactive material over a large area, but would probably not cause a large number of deaths."

The spokesman said the NRC had "no reason to believe that there is any organized effort to steal this material," but that security requirements had been tightened since the Sept. 11 attacks on New York and Washington.

The agency expects some of the material was lost or taken at construction sites where it is used to x-ray wells and test soil density, the spokesman said.

The letter from NRC chairman Richard Meserve said even a small radioactive source, if dispersed by a bomb, "could spread low-level contamination over an area up to several city blocks, possibly resulting in restriction of the area until the area was surveyed and decontaminated," but should not result in deaths.

It said varying amounts of 21 different radioactive isotopes were missing, and for some of these isotopes, the total amount missing exceeded levels considered to be highly hazardous.