WASHINGTON - The State Department said Thursday that it was "deeply concerned" by the reported discovery of depleted uranium in Colombia that may belong to a leftist guerrilla group.

"We are deeply concerned by the reports that FARC members were trafficking in uranium," State Department spokeswoman Heide Bronke said. "This underscores the terrorist threat that FARC poses to the people of Colombia and to the region."

On Wednesday, Colombian authorities dug up 66 pounds of what's presumed to be depleted uranium, which the government says the Revolutionary Armed Forces of Colombia - FARC in its Spanish initials - had attempted to obtain.

Depleted uranium is a byproduct of a process that turns ordinary uranium into enriched uranium, the material used to make weapons and nuclear energy. The dense depleted uranium, which is more widely available than enriched uranium, is used to make shells and tank armor as well as radiation shielding material and counterweights in aircraft. It has no known black-market value and is useless as a dirty bomb.

The Bush administration wants to know what motivated FARC to seek uranium in the first place. A computer file seized from a FARC camp in Ecuador that Colombian troops attacked March 1 suggests that the Marxist guerrilla group planned to sell the uranium.

"While we do not have details of the seizure, initial reports are that the uranium was depleted," Bronke said. "We have no indication at this time as to how the FARC intended to use the uranium. We commend the Colombian military for disrupting the FARC activity and hope that Colombian officials conducting the investigation will be able to determine the FARC's intended use."

The discovery of the uranium is the latest chapter to emerge from the explosive computer files belonging to slain guerrilla leader Raul Reyes. The Colombians have released only a small portion of the files in several computers and flash drives, which suggest that the FARC had broad financial dealings with Venezuelan President Hugo Chavez and contributed to the presidential campaign of Ecuadorean President Rafael Correa.

The Organization of American States has been asked to smooth over a deep diplomatic rift between Colombia and Ecuador after the March 1 incursion, which claimed 25 lives, including one Ecuadorean citizen.

OAS head Jose Miguel Insulza played down the discovery of the uranium because it wasn't the more dangerous enriched kind.

"This is not an imminent problem," Insulza said Thursday at an event organized by two Washington research centers, the Inter-American Dialogue and the Center for Strategic and International Studies. "The FARC doesn't have some kinds of missiles that other (illegal) groups have; I would doubt they have any capacity to enrich uranium. But we will look into the matter, of course."

Daryl Kimball, the executive director of the nonpartisan Arms Control Association, said the radioactivity of depleted uranium was very low.

"It is toxic when ingested. I would not let children play with it," he said. "But it is not dirty-bomb material."

The seizure raises questions about whether the source that provided the uranium to the FARC also could supply more dangerous materials or whether it was a scam to trick the leftist insurgency into believing that it was buying something more valuable. The computer files suggested that the FARC would purchase 50 kilograms - 110 pounds - of uranium at $2.5 million per kilo.

Frank von Hippel, a theoretical physicist and a professor of public and international affairs at Princeton University, said "there have been a lot of scams, with people trying to sell depleted or low-enriched uranium as highly enriched uranium, and it's not that easy to tell the difference for an unsophisticated purchaser."