Fearing an imminent dirty bomb attack, scores of nuclear scientists with sophisticated radiation detection equipment hidden in briefcases and golf bags scoured five US cities over Christmas and New Year, officials involved in the emergency effort say.

The call-up of radiation experts to Washington, New York, Las Vegas, Los Angeles and Baltimore - the first since the weeks after the World Trade Centre attacks - was conducted in secrecy, in contrast to the very public cancellation of flights from Europe and Mexico.

The terrorism crisis began late on December 19, when analysts assembled what they described as extremely specific intelligence, including electronic intercepts of phone calls and emails from al-Qaeda operatives, of an attack on US soil. A "code orange", or high alert, was issued on December 21.

Even now, hundreds of nuclear and bio-weapons scientists remain on high alert at several military bases across the US, ready to fly to any trouble spot. Pharmaceutical stockpiles to treat victims of biological attacks were loaded on transportable trucks at key US military bases.

Officials said one of their main challenges was determining whether al-Qaeda is planting provocative but false clues as a diversion or as deliberate disinformation to test the US response.

The attention to a potential dirty bomb attack resulted not from specific information pointing to an attack, but from the belief among US officials that al-Qaeda was sparing no effort to try to detonate one.

A dirty bomb attack, in which a conventional bomb is detonated and spews radioactive material and radiation across a small area, is unlikely to cause mass casualties, but could cause panic and devastate a local economy.

US officials became concerned that a large, open-air New Year's Eve celebration might be a target. While the perimeters of football stadiums can generally be secured, outdoor celebrations are much more vulnerable.

There were fears al-Qaeda operatives would hijack and crash an overseas flight into a US city or the ocean. Another was that terrorists would shoot down an aircraft with a shoulder-fired missile.

On the same day the national threat level was raised to orange, the Homeland Security Department sent out radiation detectors and hundreds of pager-sized radiation monitors for use by police in the biggest US cities.

It also dispatched the Energy Department radiation experts to cities planning large public events. In Manhattan, up to a million people were expected in Times Square on New Year's Eve.

The Energy Department scientists arrived to take covert readings on their disguised radiological equipment in a variety of settings. "Our guys can fit in a sports stadium, a construction site or on Fifth Avenue," one Energy Department official said. "Their equipment is configured to look like anybody else's luggage or briefcase."

Starting on December 22, the teams criss-crossed US cities, taking measurements 24 hours a day.

On December 29 in Las Vegas, the searchers got their first and only radiation "spike" - at a self-storage warehouse. The White House was notified.

It turned out that the locker belonged to a homeless man and, tucked inside his duffel bag was a cigar-sized radium pellet, used to treat uterine cancer. He had found it three years before. The man was released. Five tense hours after their radiation detectors had spiked, the storage locker security crisis was over.