HEN commanders gave the order for firefighters and other emergency workers to withdraw from the Pentagon crash site on Sept. 11, some heard it and evacuated. But others did not.

The F.B.I. had reported that a fourth hijacked airplane was headed for Washington and possibly the Pentagon. "We radioed the division commander to evacuate his personnel," said Edward P. Plaugher, the fire chief for Arlington County, Va., where the Pentagon is located. "And he said, `I can't, because I can't talk to them.' "

The problem was that the multitude of agencies that came to help at the Pentagon — as in New York — used such a cacophony of different radio frequencies that police and fire chiefs could not reach all the people working under their command.

The incoming plane turned out to be United Airlines Flight 93, which eventually crashed in Pennsylvania. "That aircraft didn't hit the Pentagon," Chief Plaugher said. "But what if it had?"

The Naval Research Laboratory in Washington believes that it has a way to prevent such mixups in future emergencies: the InfraLynx, a comprehensive communications command center in the form of a modified black Humvee.

The InfraLynx is designed to provide commanders at emergency scenes with land lines, cellular service, wireless Internet, fax and streaming video, and to allow emergency workers from various agencies to talk via radio. "The InfraLynx mission is to deliver emergency communications right into the hot zone," said Chris Herndon, the director of the team that engineered it at the laboratory, a part of the Office of Naval Research.

The vehicle looks like an ordinary civilian Hummer with a large box on the back and a few extras not available at your local dealership, including a cellular antenna, police lights and a yawning roof-mounted satellite dish.

Most of the InfraLynx's components are available commercially. Similar radio systems are used in many cities, and the video-processing equipment is used by television news trucks. What makes the InfraLynx so unusual is that it crams so much into a single vehicle, one that can race to the scene of a terrorist attack or natural disaster. When phone lines are destroyed, it can become the local telephone service provider for emergency workers. When cellphone networks get saturated, it can turn into a cellular tower.

Most important, it allows emergency workers from separate organizations to talk to one another. The InfraLynx acts as a radio switchboard, patching one frequency into another, bridging the signals so that anyone can talk to anyone else.

At the Winter Olympics in Salt Lake City, the InfraLynx enabled personnel from the F.B.I., the Secret Service, the National Guard and various police and fire departments to talk to one another, and at one point became a mobile dispatch center for Justice Department agents during a bomb threat at one of the Olympic sites, Park City, Utah.

Scott Behunin, the director of the Utah division of emergency services and homeland security, said, "We had 10,000 security and law enforcement personnel show up, and they all brought their own radio systems, with different frequencies, some digital, some analog.

"We don't all have Motorolas. The InfraLynx allowed us to communicate."

With its 45-foot telescoping antenna, the InfraLynx can act as a cellphone tower, emulating commercial carriers or creating a private network for law enforcement officers.

Unlike the kind of temporary cellular tower — or cell-on-wheels — that most wireless carriers deployed on Sept. 11, the InfraLynx's cellular capability does not depend on tying into local land lines. Instead, it gathers consumer cell signals and beams them to a satellite, landing them in another city.

It also carries 96 land lines that can be used to receive calls to 911 or the Red Cross when local telephone lines go down.

The InfraLynx was designed at the request of the Federal Emergency Management Agency after Sept. 11. Only three have been built so far, though the Office of Naval Research is in discussions to provide 4 vehicles to the Office of Domestic Preparedness at the Justice Department, 10 to FEMA, and 1 each to local agencies in Chicago, Washington and New York State. The vehicles are expected to cost about $800,000 each.

Civilians who see the vehicle generally become uneasy. "It's a sleek black Hummer with all this gear loaded on it," said Jeff Westley, an InfraLynx systems engineer. "We get a lot of looks. People see us roll up and they start to look a little nervous."

Emergency workers react with more enthusiasm. "When we show this to first responders, their reaction is, `We were told this didn't exist,' " Mr. Herndon said. "It's very easy to make believers out of them."