Five people going about the prosaic chores of daily life were indiscriminately shot dead in a 17-hour span in Montgomery County yesterday and Wednesday. Police said they believe that all were slain by an elusive assailant, acting alone or with an accomplice, who prowled the normally peaceful sidewalks and shopping centers of suburbia, squeezing off shots and then vanishing unnoticed.

The killings -- all single-shot, sniperlike attacks, police said -- occurred outdoors in public places in the Montgomery neighborhoods of Aspen Hill, Glenmont, Kensington and White Flint. The first victim, a man, fell in a supermarket parking lot Wednesday evening; then two men and two women were slain separately yesterday in just over a two-hour span, beginning at 7:41 a.m. All the shootings occurred in a 2 1/2-mile radius.

Police said they knew of no motives or obvious similarities among the victims. But because the five killings were carried out in a strikingly similar fashion in a county that has averaged just two homicides per month this year, police said they were convinced that the slayings were related.

"We do have someone that so far has been very accurate in what they are attempting to do, and so we probably have a skilled shooter," county Police Chief Charles A. Moose said of the gunman, who remained the focus of a law-enforcement dragnet last night.

The shootings put the terrorism-conscious Washington region on a heightened state of alert, as law enforcement agencies joined in air and ground searches for the killer and school officials took precautions to protect children. In a regionwide hunt for a white "box truck" that may have been involved in the shootings, police made numerous traffic stops of vehicles matching that description, in some cases approaching with guns drawn. But as darkness fell, the killer remained on the loose.

Ten years since the D.C. sniper shootings

Schools in Montgomery and surrounding jurisdictions went into emergency lockdown status, with students banned from outdoor activities. School officials across the region said they planned to open schools today even if the search was continuing but said they would do so with greatly increased security.

The bodies of the five victims -- three men and two women -- were taken to the Maryland medical examiner's office in Baltimore for autopsies. As of last night, pathologists had not recovered any bullets from the victims, a source familiar with the investigation said.

Police said last night that they were not certain what type of weapon was used, although they said it may have been a powerful rifle, and added that no spent shell casings had been found. And they were unable to find eyewitnesses to the killings, even though yesterday's attacks occurred in well-traveled areas during the morning commute.

"We are certainly not closing any door," Moose said of the investigation, adding that police are considering all possible motives for the slayings. "We are taking a broad-brush approach to this."

Although police searched vigorously yesterday for a slightly damaged white truck with two occupants that a witness saw racing from one of the shooting scenes, at the Leisure World Plaza Shopping Center in Silver Spring. Police said only one witness saw the truck with two occupants leaving the scene shortly after a shot was fired, but the witness did not see a shot come from the truck. The truck has not been found.

Police departments across the country were alerted, and federal and state law enforcement agencies were called in to huddle with county police last night.

Schools were temporarily locked down across the area yesterday, authorities said, while frantic parents telephoned to check on their children's safety. Schools let out on time, but many had police on guard.

County Executive Douglas M. Duncan (D), who rushed home from a visit to Chicago, said that public schools would be open today.

Across the county, fearful residents and workers stayed indoors, and some streets and public places appeared deserted.

Investigators said that all the victims died where they were shot, in a four-mile-wide cluster just east of Rockville. Two were at service stations -- one buying gas, another vacuuming a minivan. A third was in a supermarket parking lot. A fourth was sitting on a bench outside a post office, and the fifth was mowing grass near a car dealership.

"They're shooting all around!" said a frightened bystander who happened on one of the grisly scenes yesterday morning.

The slayings shattered the fall morning amid the early bustle of the workday, as, one by one, dire reports flashed to police at almost half-hour intervals between 7:41 and 9:58 a.m.

Investigators quickly linked yesterday's four slayings to a fifth Wednesday evening, as well as to a shooting that punctured a store window late Wednesday afternoon, the incident that they believe began the shootings.

The attacks left scenes of horror to be discovered by stunned passersby and morning commuters on a gorgeous morning.

"It was beautiful, it was perfect, it was an ideal morning," said a man who found one of the victims dead in a White Flint parking lot with a gaping gunshot wound to the chest. "That's the part that's strange."

Officials said they believe the shootings began at 5:20 p.m. Wednesday, when someone fired a shot through the window of Michaels, an arts and crafts store, in a shopping center at Georgia Avenue and Aspen Hill Road. The store was busy, but no one was hit.

Assistant Manager Debbie Kay said she heard what sounded like a light bulb pop and then noticed a marble-size hole in the display window and called police.

Forty minutes later and two miles south, 55-year-old James D. Martin was shot while crossing the parking lot of Shoppers Food Warehouse in Wheaton's Glenmont Shopping Center, at Randolph Road and Georgia Avenue.

Martin, who lived in the county's Stonegate neighborhood, north of Colesville, was a program analyst for the National Oceanic and Atmospheric Administration's Office of Marine and Aviation Operation. He was married and had an 11-year-old son.

The shootings resumed yesterday at 7:41 a.m.

James L. "Sonny" Buchanan Jr., 39, who operated a landscaping business, was mowing a strip of grass along Huff Court behind Fitzgerald Auto Mall in White Flint when he was shot once in the torso, according to police and witnesses.

Buchanan had a wound "as wide as a coffee cup," said Richard Tyner, 50, of Germantown, a local office worker who was one of the first to reach him.

"I just put my hand on his shoulder and said, 'Help is on the way,' " said Al Briggs, dealership service director. "But he was already gone."

A distraught Tyner, who said he saw no commotion and no one running away, added later: "If I had gotten there 15 seconds earlier, I might have been able to do something -- get a license number, get a description. . . . I would have done everything in my power."

Thirty-three minutes later, at 8:12 at a Mobil station at Aspen Hill Road and Connecticut Avenue, three miles north of where Buchanan was shot and a block from where the store window was shot the night before, Premkumar A. Walekar, 54, an Indian immigrant from Olney, was shot while filling up his cab's gas tank.

Walekar stumbled into a minivan parked at a nearby gas pump and then fell to the ground. "He's been shot! He's been shot!" the minivan driver shouted. She put her left hand under his head and began holding his hand.

The woman then started CPR. "She must have been a nurse or something. That woman was on the money," said Alex Millhouse, a station mechanic.

Police arrived in minutes. "He doesn't have a pulse," the woman told them. "He's not breathing."

Twenty-three minutes passed.

At 8:37, two miles north, Sarah Ramos, 34, of Silver Spring was sitting on a blue metal bench outside the post office at Leisure World, at Georgia Avenue and International Drive, her purse resting beside her, when she was shot in the head.

Again, nobody saw much. Wearing jeans and brown shoes, Ramos had been sitting with her legs crossed, perhaps waiting for a friend or a ride.

"God, it could have been me -- I was on my way to the post office," said Dan Ranhart, 69, a resident of the Leisure World retirement community, as he happened upon the spot clutching a small pouch of letters.

Police had covered Ramos with a sheet, but her body still sat on the bench three hours after her slaying. It was removed about 11:30 a.m.

"I don't want to go home. I'll stay put right here," said Alma Wilson, 58, a caregiver at Leisure World who was on a break from work. Her brown eyes opened wide as she glanced at a nearby bench. "I'm scared to sit on a bench," she said.

Hairdresser Suzy Cooper, 57, who works at Images Hair Design near the post office, said she had been in the back of the salon, drinking coffee, when she heard a popping sound. She thought that someone had thrown something, but then a woman rushed into the salon, crying and yelling for police.

But it was not over.

The killer headed back south, amid the thinning inbound morning rush and the spreading public realization of the morning's disaster.

At 9:58, at a Shell gas station at Connecticut and Knowles avenues in Kensington, five miles south of Leisure World, Lori Lewis Rivera, 25, stopped to vacuum her minivan. Moments later, service station attendants found her collapsed inside the van, pinned by the driver's side door and bleeding.

The shootings were reminiscent of a series of attacks that terrorized two District neighborhoods for more than six weeks in 1993, when the "Shotgun Stalker," as he became known, killed four people and seriously wounded five others.

The assailant, James W. Swann Jr., then 30, drove around the Northwest Washington neighborhoods of Mount Pleasant and Columbia Heights and fired 20-gauge shotgun blasts at the heads of lone pedestrians, mostly at night.

He was arrested minutes after his last attack on April 19, 1993. He was later found not guilty of murder by reason of insanity and confined to a mental hospital.

The killings yesterday and Wednesday brought the number of homicides in Montgomery County to 25 for this year. It is the highest number since 1997.

The last time five people were slain in a 24-hour period in Montgomery was in July 1995 in Potomac, in one of the worst criminal cases in recent Maryland history. A handyman's assistant killed podiatrist David Marc Goff, his three daughters and a contractor at Goff's home.

Bruman Stalin Alvarez, then 21, pleaded guilty and was sentenced to six consecutive life terms, five without the possibility of parole.