WASHINGTON (AP) – Federal investigators say the government’s next generation radiation detectors are only marginally better at detecting hidden nuclear material than monitors already at U.S. ports, but would cost more than twice as much.

The machines are intended to prevent terrorists or criminals from smuggling into the U.S. a nuclear bomb or its explosive components hidden in a cargo container.

The monitors now in use can detect the presence of radiation, but they cannot distinguish between threatening and nonthreatening material. Radioactive material can be found naturally in ceramics and kitty litter, but would be of no use in making a bomb, for instance.

The Department of Homeland Security has said the new machines it is developing can distinguish between kitty litter and dangerous radioactive material and produce fewer false alarms than the current ones.

The new one are also better at detecting lightly shielded material. But the machines perform at about the same level when detecting radiological and nuclear materials hidden in a lead box or casing, the most likely way a terrorist would try to sneak the materials into this country, the Government Accountability Office said in a report to be released Monday.

The report by Congress’ investigating agency was requested by Sen. Joseph Lieberman, the Connecticut independent who is chairman of the Senate Homeland Security and Governmental Affairs Committee.

The report raises the question whether the new machines, at about $822,000 each, are worth the cost if they’re not that much better than current ones that cost about $308,000.

The department believes the benefits outweigh the additional cost. But department spokeswoman Sara Kuban said Friday the department would not comment specifically on a report that was not yet public. She said the new machines still are being tested.

In March, Homeland Security Secretary Janet Napolitano told Lieberman’s committee that the administration is not asking for money to purchase these machines in 2010.

“The reason is because we were not persuaded that the technology … the capacity of the technology that we needed, was actually there,” she said. “Before we come to Congress and ask for more money for new technology, we needed to see something better from the science community.”

For years, Congress and the GAO have been skeptical about the cost and testing of the new machines. The new report did not criticize the department’s latest tests, but found the results disappointing.

The new monitors “have a limited ability to detect certain nuclear materials at anything more than light shielding levels,” the report said.

The Senate’s spending plan for the department in 2010 calls for investing money to improve the current machines as well as hand-held radiation detectors now in use. Because the current technology will be used for some time, “it would seem prudent to invest in this technology,” the Senate Appropriations’ homeland security subcommittee concluded.

The acting head of the department’s Domestic Nuclear Detection Office, Chuck Gallaway, recently told Congress that the new machines “will significantly improve our ability to correctly identify and interdict smuggled nuclear material and offer the ability to automatically sort threat materials from naturally occurring radioactive material.”