NORTH LAS VEGAS, Nevada (AP) — In an unremarkable building a short drive from the Las Vegas strip, government analysts hover over computer monitors, watching waves of color sweep over a map of a city.

The city is Washington, D.C. The hues represent the fallout from an imagined nuclear bomb.

It is from here, in a laboratory on the edge of the vast Nevada desert, that U.S. officials would gather some of the first critical information that could affect the lives of millions in the aftermath of a nuclear terrorist attack in an American city.

Normally concealed from the world within the high fences of Nellis Air Force Base, the doors were opened last month to provide rare tours for officials from 26 countries. A reporter from The Associated Press was also invited along.

The tours were part of efforts by the National Nuclear Safety Administration to coordinate international responses to nuclear or radiological catastrophes in the United States or abroad. The visiting countries included China, Russia and Israel.

"We have great concerns about terrorists acquiring a nuclear weapon or a dirty bomb," says Vince McClelland, who heads NNSA's office for international emergency management and cooperation. "The intent is to work with as many countries as possible to ensure they have systems and programs in place so that if something happens we can assist each other."

There is, of course, another benefit for the United States in offering its technology and expertise in an atomic emergency.

"When they request help, we know there is a problem," McClelland said.

What the visitors saw was the technology that would be used to rapidly assess any significant release of radiation in the United States, or if asked, abroad. This could be a nuclear detonation, a radiological "dirty" bomb or an accident at a reactor.

In the case of a major release of radiation the laboratory is prepared to act as a control room to collect all the information on the catastrophe.

From the laboratory at Nellis, and a similar facility at Andrews Air Force Base outside Washington, analysts would assess the strength of a blast, the degree of radioactivity and the area of fallout.

In the control room, lined with monitors from wall to wall, analysts would assemble the information to help officials ranging from local mayors to the president make life-or-death decisions about how to respond, who should be evacuated and where they should go.

The first details would be known within moments. The NNSA can provide rough estimates of fallout following a detonation based on weather patterns and the apparent size of the blast.

Planes and helicopters equipped with detection devices would be dispatched immediately from Nellis or Andrews capable of reaching any part of the continental US within hours. They would transmit blast and weather data to the control rooms.

On the tour, analysts demonstrated how they would use that information to produce detailed maps of the blast, illustrating ground contamination and how much exposure to radiation people had in various locations.

They would get more data once it is safe to deploy people on the ground. Medical experts trained in treating radiation would work with local doctors. Communications specialists would set up their own network, so they won't be reliant on potentially overloaded local networks. Field laboratories can send radiation samples from the air, soil and the skin of victims to the control rooms.

At Nevis and Andrews, small teams are on call around the clock ready to ship out by plane in an emergency. Such response teams could be expanded to include hundreds of medical, radiation and communication experts with mobile laboratories if necessary.

Specialists can also assess data for clues about the perpetrators, determining the likely origin of nuclear material and design of the explosive. Nuclear data can sometimes pinpoint the material down to particular uranium mine and nuclear program.

NNSA officials emphasize the importance of speed in their response.

"We are prepared to respond to nuclear emergencies in a moment's notice 24 hours a day," says Joseph Krol, who heads NNSA's emergency operations. "Our responsibility is to provide responders with critical and timely information."