WASHINGTON (AP) - The government should tighten licensing procedures for radioactive materials, according to a group of outside experts assembled after a congressional investigation identified security gaps.

In a probe last year that set up a bogus company, investigators said they were able to obtain a license from the Nuclear Regulatory Commission that allowed them to buy enough radioactive material for a small "dirty bomb."

A panel of independent technical experts that examined the licensing process for potential vulnerabilities has recommended several changes:

-Officials should visit a new applicant's facilities before issuing a license.

-The NRC should conduct background checks.

-The agency should suspend its "good faith presumption" that new applicants seeking radioactive materials are honest.

An NRC spokesman, Eliot Brenner, said the agency had put those changes into effect even before the report March 18 from the review panel.

"We went from a trust-but-verify, to a no-more-Mr.-Nice-Guy attitude," Brenner said Wednesday.

Sen. Norm Coleman, R-Minn., said the government must be vigilant in regulating and tracking radioactive materials.

By making the recommended changes, the NRC "has shown it understands the importance of ensuring that the nightmare scenario of a dirty bomb attack never occurs by preventing America's enemies from acquiring radiological materials in the first place," Coleman said in a statement.

Coleman and Sen. Carl Levin, D-Mich., had asked the Government Accountability Office to investigate the issue. Levin is chairman of the Senate Homeland Security and Governmental Affairs Permanent Subcommittee on Investigations and Coleman is the panel's top Republican.

Coleman had said that nobody at the NRC checked whether the bogus company was legitimate and an agency official even helped the investigators fill out the application form.

The license that was obtained allowed for the purchase of up to five portable moisture density gauges widely used in construction, in which are encased small amounts of cesium-137 and americium 241, two highly radioactive isotopes.

Individually, these devices pose little threat because of the small amount of radioactive material, radiation experts have said. Still the devices require an NRC license to be purchased and must be closely safeguarded by companies that use them to avoid theft.

But the investigators found a way to purchase as many as 45 of the gauges and could have bought many more because they duplicated the NRC-issued license and removed the restrictions on the amount that could be purchased.

A dirty bomb could spread radiation by a conventional explosion but would not have a nuclear detonation. Experts believe such a bomb would not cause casualties beyond those affected by the explosion. But such an attack could have significant psychological impact and have serious economic consequences because of cleanup problems.

The investigators never finished the deal because they did not have the money to buy the machines - which cost about $5,000 apiece - and did not have a place to store them safely.