Emergency responders cross county lines every day in the greater Washington region, and their radios go along, too. But some emergency communications officials fear that a reshuffling of the radio spectrum will threaten their ability to communicate across borders.

In the late 1990s, police and fire agencies started to notice interference on their radio channels from such commercial users as Nextel. In response, the Federal Communications Commission decided last year to authorize a nationwide swap in which public safety frequencies would move to the lower end of the 800-megahertz band and commercial users would move to the higher end. This three-year, $4.9 billion project began June 27 and will be paid for by Nextel.

The project involves moving thousands of police and fire radio systems to new frequencies, and Tony Rose, Charles County's emergency communication chief, worries that in the short term it will cripple the ability of emergency responders to talk across borders.

"When this is all said and done, rebanding will be one of the best things done for public safety in a long time," Rose said. "But it's getting there that's the problem."

For example, Fairfax County is slated to change its system months before many neighboring counties. "There are 15 jurisdictions and 40,000 radios," he said. "Do these 40,000 radios not talk to Fairfax [when it changes over], or do we [reprogram] them once so they can talk to Fairfax and then once again to talk to Prince William?"

Grossly simplified, the problem can be somewhat equated to one friend changing his cell phone number and another reprogramming his new number in her own phone. It's as if 15 friends change their numbers, and each person has thousands of phones to reprogram. Having a technician reprogram each radio to recognize a new frequency each time a neighboring county switches could be expensive; not changing them could cost lives if a firefighter responding to a blaze in a neighboring county could not use his radio.

"The question is how many first responders are placed at risk because interoperability is down?" Rose asked.

Officials at Nextel and the Transition Administrator, the independent entity created by the FCC to oversee the rebanding, said there will be no lapse in service.

"We don't anticipate that there should be any situation when mutual aid is off the air," said Joe Boyer, a member of the Transition Administrator team. He said new technology coming from Nextel would make it easier to switch the channels on the region's tens of thousands of radios. Anne Arundel County and Alexandria police say they already have plans to use a technology that can connect seemingly incompatible radio systems.

Boyer said the specific logistics of the switch cannot be worked out until each county starts negotiating with Nextel in the coming months. "The reason we can't say, 'This is how you have to do it,' is because every area of the country is different," Boyer said.

Tim O'Regan, a spokesman for Nextel, said early planning and cross-county coordination would be critical in preserving the ties. "When we get around to negotiating with public safety, part of our discussions with them will be to raise this matter. Do you have mutual aid channels? What are those? Have you talked to those people about the fact that you're about to be reconfigured?"

Supporters and critics agree that what makes the transition so difficult is the same thing that makes the plans for it difficult to judge: Nothing this big has been carried out before.

Maryland, Virginia and the District are all in the first wave of the rebanding project, but each wave is divided into two stages, and most of the public safety systems are in the second stage.

Fairfax has the distinction of being in the first stage and will begin negotiating with Nextel within weeks. The early changeover date means the county is at risk of falling out of step with its neighbors, which worries James Wadsworth, manager of the Fairfax County Radio Services Center.

"This is the most significant thing that's ever hit the radio waves, ever in the history of radio. It's coming at us like a freight train," he said.

After meeting last month with representatives of Nextel, the FCC and the Transition Administrator, Wadsworth was even more worried.

"They said there would be no interruption of service," he recalled. "And I said, 'Okay. Why are you only talking to Fairfax in this phase? Because all the other jurisdictions would be affected when we change.' And they didn't have an answer."

He is not alone in saying planners vastly underestimated the magnitude and complexity of rebanding the nation's public safety radio systems.

Members of the Metropolitan Council of Governments' police communications subcommittee are anxious not only about their ability to communicate across borders, but also about the tight deadline and daunting logistics.

When Montgomery County joined the 800-megahertz system in July 2003, it was a year behind schedule, said subcommittee Chairman Alan Felsen, who works for the Montgomery County police's technology division. If the problem of operating across borders is going to be solved by every county's switching over on the same day or week, as some have suggested, such delays could tear up even the most coordinated project, especially when amplified over a dozen or so jurisdictions, he said.

"Even if you could say this is all going to go perfectly, I don't understand how we can preserve the level of interoperability during the process," Felsen said.

But Robert Gurss, director of legal and government affairs for the Association of Public-Safety Communications Officials, said that once planners sit down "with pencil and paper," they will be able to work through the problems of organizing the switch. "This is not something that everyone woke up eleventh-hour and realized there was a problem," he said.

And Capt. Tim Bowman, of the Anne Arundel police department, said his agency had already made plans to reprogram all the radios that need to be fixed in stages according to their priority. "What it becomes is an inconvenience," he said. "We have to pull them in and touch them. It's not hard, it's just that there are so many of them."