SILVER SPRING - A tip about a white truck was the best lead police had last night in a string of brazen, sniper-style shootings in the heart of Montgomery County that left five people dead and swept the suburbs with a wave of fear.

The shootings yesterday morning and Wednesday night - carried out with deadly accuracy with what police say may have been a high-powered rifle - set off a huge manhunt and threw the county into high alert as police urged motorists to stay off the roads, schools shut students in classrooms and parents driving children home had them crouch in their seat.

The shootings occurred within a few miles around Wheaton and along the county's busy thoroughfares. Killed were a 55-year-old Colesville man outside a grocery store; a 39-year- old Abingdon, Va., man mowing the lawn at a car dealership; a 54-year-old Olney cabdriver refueling his taxi; a 34-year-old Silver Spring cleaning woman sitting on a bench near a post office; and a 25-year-old Silver Spring woman vacuuming her car at a gas station.

A witness saw two men speeding away from the post office in a white box delivery truck with black lettering and a damaged rear lift gate. Police said the truck was probably an Isuzu. Hundreds of state, city and county police, as well as FBI and other federal agents, were pulling over white vans and trucks across the state last night, without finding suspects.

To police and residents alike, every aspect of the shooting spree was chilling: the apparent randomness with which victims were selected; the daring of choosing such public places for the attacks; and the precision of the hits, each apparently with a single shot.

"We have someone who, so far, has been accurate in what they are attempting to do," said Montgomery County police Chief Charles Moose. "We feel like we have a skilled shooter, and that heightens our concern."

The incidents began, police say, with a single, puzzling shot that gave no hint of the carnage to follow. At 5:20 p.m. Wednesday, a bullet hit the lighted number over one of the cash registers at Michaels craft shop on Georgia Avenue. At first, workers thought it was a power outage; then they saw the bullet hole in the window.

A clerk was ringing up a purchase at the time.

"I would have been No. 1," she said.

Officers searched displays of flowers and ribbons for hours, looking for the bullet. As far as police know, it was the only one to miss its target.

About 45 minutes later, James D. Martin, 55, of Colesville, was gunned down in the parking lot of a new Shoppers Food Warehouse on Randolph Road in Wheaton. Police were reviewing tapes from surveillance cameras.

The violence resumed early yesterday, when James L. "Sonny" Buchanan, of Abingdon, Va., 39, was shot while mowing the lawn outside Fitzgerald Automotive in the 11000 block of Rockville Pike.

An employee heard a loud bang around 7:40 a.m. - loud enough to cause him to drop to the ground - then saw Buchanan running along the rear fence before he fell, said Al Briggs, the dealership's service director.

Briggs drove up soon after and ran over to Buchanan's bloodied, unconscious body. He said he knew immediately that the man wasn't going to make it, but told him help was on the way.

"The whole time this morning I was hoping against hope it wasn't him," said Rob Smith, the dealership's service manager, a friend of Buchanan's.

The dealership gave video from surveillance cameras to police.

About a half-hour later, Premkumar Walekar, 54, of Olney was refueling his taxi at a Mobil station at Aspen Hill Road and Connecticut Avenue, as he does almost every day.

Alex Millhouse, a mechanic there, was sitting at a pay phone when he heard what sounded "like a double explosion, like a car backfiring." He saw Walekar clutching his left side, with blood streaming. By the time he reached him, Walekar was slumped against a van.

Millhouse and fellow mechanic Warren Shifflet watched as a woman from the van administered mouth-to-mouth resuscitation before an ambulance arrived. "She was shaking," Shifflet said.