British officials believe that Usama bin Laden's Al Qaeda terrorist network successfully built a crude radiological device known as a "dirty bomb" in Afghanistan, the British Broadcasting Corp. reported Thursday.

British intelligence agents found documents that showed Al Qaeda members had built a small device near Herat in western Afghanistan, the BBC said, citing unidentified British government officials.

Britain's Foreign Office said Thursday the report substantiated expert opinion that Al Qaeda wanted to develop a nuclear weapon.

"The evidence presented in the BBC report speaks for itself," a spokesman told The Associated Press. "It provides proof to substantiate expert opinion that Al Qaeda was interested in developing nuclear weapons."

In Washington, a U.S. official, speaking on condition of anonymity, said bin Laden was no doubt interested in acquiring a "dirty bomb" -- a conventional bomb capable of spreading radiation.

But the U.S. official said, "We have no evidence to substantiate that he's built such a device."

The British intelligence agents did not find the device itself and it has not since been recovered, BBC reported.

But scientists at the British government's weapons research facility in Porton Down concluded that Al Qaeda had succeeded in constructing a small "dirty bomb" in Herat -- based on documents and material uncovered by the British military and intelligence, the BBC said.

The scientists did not believe Al Qaeda had been able to develop a full-blown nuclear device, it said. The report did not say when the device was thought to have been developed or how much radiation it could spread.

British officials showed some of the documents -- including diagrams -- to the BBC, the report said.

As part of the operation, British agents infiltrated Al Qaeda camps in Afghanistan, posed as recruits and reported back, BBC said.

The British officials told the BBC that Afghanistan's Taliban regime helped Al Qaeda construct the device by providing medical isotopes.

There has been previous evidence of Al Qaeda's interest in a "dirty bomb," which would be far less deadly and damaging than a nuclear explosion.

Computers found by journalists and U.S. troops at a variety of facilities in Afghanistan indicated Al Qaeda had sought to obtain and develop nuclear and other potent weapons.

During a New York trial two years ago, a former bin Laden aide testified he was ordered in 1993 to try to buy uranium on the black market for an effort to develop a nuclear weapon. Jamal Ahmed Al-Fadl said Al Qaeda was prepared to spend $1.5 million, but he didn't know if a purchase was ever made.

In addition, U.S. officials have said captured Al Qaeda lieutenant Abu Zubaydah told American interrogators the terrorist network was working on a "dirty bomb."

Authorities also have said that Jose Padilla, a former Chicago gang member charged with plotting with Al Qaeda, attended two meetings in Karachi, Pakistan, at which senior Al Qaeda operatives discussed the possible use of a "dirty bomb."