Northern Virginia police were involved in an intensive search in Prince William County last night and early this morning for someone they believe to be a law enforcement officer who put out a radio transmission saying he was being shot at during a pursuit.

The search was focused on an area between Route 1 and the Occoquan Bay, officials said. In addition to Prince William patrol officers, Fairfax County police and Virginia State Police helicopters and an FBI airplane were searching, along with marine police.

The search was touched off by a radio transmission received by Prince William police at 6:50 p.m., Officer Kim Chinn said. The radio call came from a man who said he was pursuing someone in his car on Interstate 95 near Woodbridge, officials said. The caller never identified himself, said Chinn.

Chinn said sirens were heard in the background as the man said "we're in pursuit," then added after a pause, "there's automatic weapons fire." Police then lost voice contact with the man.

A short time later, radio contact was reestablished, and the man communicated by answering questions with radio clicks, Chinn said.

Police have not ruled out the possibility of a hoax, and early this morning said that was a growing suspicion.

In addition, she said the Federal Communications Commission has been called in to try to locate the signal, which came in on Channel 2, one of three frequencies used by Prince William police.

Police early this morning set up a command post at Tyme N' Tyde, a private boat dock on the Occoquan Bay, close to the location they believe the call originated from.

Chinn said the last communication from the caller was between 11:30 and 11:45 p.m.

Police were treating the matter as a law enforcement officer in trouble, Chinn said.

Before the search was called off about 2 a.m., two police helicopters circled a wooded area near the bay early this morning as Prince William police Lt. J.C. Johnson radioed at 30-second to one-minute intervals in continuing efforts to contact the caller.

Johnson had been in contact with the man since shortly after the first call, police said. Police decided not to resume the search this morning.

Officials said that all on-duty officers from the Prince William Police Department and the Virginia State Police were accounted for last night.

Police did not know what police agency the man might have been associated with, Chinn said.

She said several local police departments and the Drug Enforcement Administration use the same radio frequency as the Prince William police.

Prince William police asked anyone with information to call (703) 335-6500.