Vernon Garrison was stuck at the supermarket -- outside, actually, in the front seat of a navy blue Caprice Classic that would not start.

His keyless entry remote control, the device that unlocks the doors and also is required to start the engine of his car, had gone haywire again. "I was furious," said Garrison, 67, who owns a gas station and, fortunately for him, another car.

For 20 minutes as he waited for help, Garrison watched keenly as several customers came out of the Shoppers Food Warehouse in Waldorf, aimed remotes at their cars and failed to open their doors. Point, push, nothing. Just as he thought.

"I could see them walk up," he said, remembering that afternoon two months ago. "I'm saying, 'It's not going to work.' "

Matt Drake has seen this, too, and wishes he had an explanation. He works at a Radio Shack across Route 301 from the grocery store.

"Volvo, BMW, Mercedes, it does not discriminate," Drake said, pointing over the counter to the strip mall parking lot. "If every single one of those cars has a keyless entry, every single one will not work."

The sporadic incidents -- at least five days in the past year, by Drake's count -- have become something of a mystery in Waldorf, a sprawling mix of shopping centers and subdivisions in Charles County. But such outages are not unprecedented.

Three years ago, thousands of drivers in Bremerton, Wash., were stumped on two occasions when their push-button remotes proved impotent. It happened in Las Vegas in February, prompting hundreds of calls to car dealerships and locksmiths. And in May, a two-way radio system being tested at Eglin Air Force Base in the Florida Panhandle jammed remote control garage door openers in communities near the base.

In most cases, remote control failure is little more than a curiosity, as drivers can simply use their keys to unlock the doors. Some cars, however, require the device to deactivate an alarm or start the engine. Charles Vernon, a retiree from Accokeek whose remote first malfunctioned at the mall in Waldorf on May 10, said the problem is a safety issue and an inconvenience.

"You don't buy the car not to be able to use it," he said.

There is no shortage of speculation on what is causing the problem in Waldorf. Diana Rucci, the operations director at Waldorf Ford, overheard customers in a local restaurant saying NASA satellites were involved. Others pointed to storm clouds, cell phones, solar flares, the Taliban.

Drake said he's not so sure. "Everybody thinks it's the government," he said with a sly grin. "I think it's aliens."

Tony Rose, Charles County's chief of emergency communications, has heard about the problem but would hazard only a vague guess. "It may be something new in Waldorf, some kind of wireless technology," he said. "But I can't confirm it."

Keyless entry remotes have become standard in new cars in recent years. Of the more than 14 million cars and light trucks produced in the United States last year, 77 percent came with the remotes, up from 32 percent in 1996, according to industry research company WardsAuto.com.

The technology is similar to that used in garage door openers and remote-controlled toys. The remote acts as the transmitter, sending an encrypted message on a weak radio signal to the receiver in the car, which decodes the message and activates door locks and other functions.

But unlike other more powerful radio signals, keyless entry remotes are not licensed by the Federal Communications Commission. They are allowed to operate on frequencies used by licensed customers as long as their signals are sufficiently weak and don't interfere with others. But because of this outlaw status, their own signals can be jeopardized.

"Car entry systems, they have no rights at all," said Bruce Romano, who works in the office of engineering and technology at the FCC. "If they get interference, that's too bad for them."

Interference can occur when a stronger signal on the same or similar frequency overwhelms the receiver, and the low-powered message from the remote cannot be "heard." An engineer compared the situation to trying to have a conversation at a stock car race, where the roar of the vehicles will drown out the voices.

"In all likelihood, the disturbance is probably some fairly high-intensity signal that is only radiating at certain times, and therefore, it makes it very difficult to locate where the source of the problem is, because it's not always up," said John Daher, a research engineer who studies electromagnetic effects on electronic systems at the Georgia Tech Research Institute in Atlanta.

Some of the devices that have failed in Waldorf operate on a frequency of 315 megahertz. Another common keyless entry frequency is 302 MHz. Both of these frequencies fall within a range licensed primarily for use by the military and the federal government.

In a summary of radio spectrum use from the National Telecommunications and Information Administration, the frequencies in the range from 225 MHz to 328.6 MHz "are heavily used worldwide for critical military air traffic control and tactical training communications." Specific functions include "air-ground-air communications for combat weapons training carried out at and in the vicinity of all major air bases and military training areas worldwide."

"High-powered transmissions in this spectrum from presidential and military aircraft are thought to be responsible for interference," said Mike Swanston, a spokesman for the Consumer Electronics Association. "But the government doesn't have to say what they're doing or how often."

Military radio signals have been implicated in other frequency mishaps in recent years. In Bremerton, the home of the Puget Sound Naval Shipyard, the first outage occurred about the time the USS Carl Vinson returned to port. Less than a month later, the second failure of keyless entry corresponded with the arrival of another aircraft carrier for maintenance, according to news reports. A subsequent FCC investigation could not prove the cause of the outages.

Manufacturers of keyless entry systems say they have been battling interference for years with little recourse. Engineers from electronics manufacturer Lear Corp., based in Southfield, Mich., have made several trips across the country to sites where disturbances have been reported. In the past several years, said Tom Tang, an engineering manager at Lear, manufacturers have improved the keyless entry technology by narrowing the range of signals that can reach the receiver, in effect closing the door on unwanted interference. But an identical or very powerful signal still can foul up the keyless entry.

"These devices by their nature, and because of FCC rules, have to accept harmful interference," said John Dicroce, a product development manager at Audiovox Electronics Corp. in Hauppauge, N.Y. "We can't control any FCC approved devices."

About 15 miles north of Waldorf is Andrews Air Force Base in Prince George's County. Officials there said they have not heard about the disturbances, nor do they have an explanation for them.

However, in Waldorf, along Old Washington Road, a 300-foot-tall AT&T tower built in 1955 has several microwave dishes and a blinking white light affixed to it. The primary function of the tower, known as a "point of presence," is to coordinate local and long distance telephone service, as well as cell phone service. But on certain days, said operations manager Philip Clark, the tower also sends out a different signal.

Clark would not specify the purpose of the signal or when it is used. Another employee, who spoke on condition of anonymity, said the signal was related to "government" work.

"I've had problems with [my keyless entry] also," Clark said. The disturbance "is probably coming from our transmitter."

Clark, who works with AT&T Government Solutions, stood outside what appeared to be a windowless concrete building in front of the tower. The glass front door of the building opened into a small entryway, which led to a second white door. On the wall of the entryway was a circular convex mirror. Hanging next to it was a surveillance camera.

"We don't have a schedule when we use that signal; it occurs when necessary," Clark said. "I think it will go on . . . but we can see about not using that frequency."

A security keypad hung to the left of the white door. As Clark spoke, he smiled. "I didn't realize it was disturbing other folks," he said.

Some suspect this AT&T tower in Waldorf could be the source of a signal that affects keyless remote controls.Charles Vernon says malfunctioning remote controls are a safety issue and an inconvenience. "You don't buy the car not to be able to use it," he says.