Prince William County police are investigating a phantom radio call for assistance on a police channel Saturday that touched off an intensive seven-hour search until federal, state and local officers finally decided it was a hoax.

"A lot of people had doubts early on, but you have to follow through," said Prince William County Deputy Police Chief J.K. Sullivan, who called off the search at 2 a.m. yesterday. "At this point, it appears to be a hoax."

Sixty police officers, many of them off duty, rushed to the eastern end of the county to help, and three helicopters whirled overhead, flashing lights into the heavily wooded area near the police command post at the Tyme n' Tyde Marina in hopes of finding a clue.

The episode began when a Prince William County dispatcher received a 6:50 p.m. call Saturday from a man claiming to be involved in a chase on Interstate 95 near Woodbridge. "We are in pursuit," the caller said. "There is automatic weapons fire." Sirens could be heard in the background.

Dispatchers interpreted the call to be a request for assistance from a police officer and cars were sent out to look for the officer. The call was received on channel 2, one of three frequencies used by Prince William police and the main channel for officers on the east end of the county, although the man never identified himself as a police officer.

The dispatch center lost voice contact with the caller within seconds after the initial statements. But after asking him to get in touch, a series of clicks were heard on the frequency, leading police to believe they were responses from the mysterious caller.

Searchers included officers from Prince William County, Fairfax County, the Virginia State Police, the FBI, the Drug Enforcement Administration and a Marine police agency.

"The only reason I'm out here on my day off {is} to help an officer who might be down," a police officer who declined to give his name said Saturday. "He could be shot, he could be disabled, someone could be holding a gun on him somewhere right now."

In the early hours of the search, Lt. J.C. Johnson kept in touch with the caller via radio asking him questions to help fix his location. Clicking to indicate right answers, the caller identified his approximate location as the Veterans Memorial Park/Belmont area near the Fairfax County line just off Occoquan Bay.

Police intensified the search in that area as the caller kept up a steady litany of clicks.

Officers huddled around a police radio hanging on Johnson's every word as he spoke to the caller. Then about 11:30 p.m. the clicks -- which had been heard regularly since about 7 p.m. -- abruptly stopped. No more signals from the caller were heard despite pleas from Johnson at 30-second to one-minute intervals, asking the caller to resume contact.

As the night progressed, Johnson's voice grew slightly edgy. He last spoke to the caller about 1:45 a.m. "We're trying to help you, so you need to let us know where you're at."

"Let me hear from you," Johnson said. "Click your button if you're okay." No response. Moments later: "We're close by but we just don't know exactly where . . . . You need to let us know where you are. Click your button." Again, silence.

An FBI airplane specially equipped to intercept radio signals had been brought in to try to find the origin of the call. A state police helicopter with special signal-locating equipment was also brought in. But no signal could be picked up.

As the search was terminated, about 30 officers gathered at the marina parking lot looked tired as they piled into cars. One officer said yesterday he would come out again if the search were resumed. "At this point I don't think a decision has been made, but if they get some information that an officer is out there they'll search again and I'll be here."

Sullivan said the search had "wasted a lot of time" and cost taxpayers a considerable amount in fuel, police time and equipment costs.

He said faking a call for assistance and transmitting on a police frequency without a license are violations of Federal Communications Commission regulations. The channel is shared by Prince William County police as well as officers from Dumfries, Occoquan and the FBI, county police spokeswoman Kim Chinn said.

Chinn said police believe all their hand-held radios are accounted for. The inventory is checked about once a month, police said.

A tape of the caller's voice is expected to be analyzed by the FBI.