ROCKVILLE, Md. -- Police linked a sixth death to the sniper killings of five Maryland residents and said Friday the same high-powered rifle was used to kill at least four of the victims.

Police were searching for two men -- a shooter and a driver -- in the slayings and investigating whether a seventh shooting outside a Virginia store was part of the same terrifying crime spree.

"We implore him to surrender, stop this madness," Montgomery County Police Chief Charles Moose said.

The sixth victim, a 72-year-old Washington, D.C., pedestrian, was killed by the same weapon used to kill at least three of the Maryland victims, said Special Agent Michael Bouchard of the Bureau of Alcohol, Tobacco and Firearms. Forensic testing was still under way in the two other Maryland shootings.

Bouchard also said ATF agents would examine evidence collected from the scene of a Friday afternoon shooting outside a crafts store in Fredericksburg, Va., 55 miles south of Rockville. A 43-year-old woman was shot in the back and in serious condition.

Police were looking for two men in a white van with dark lettering, a description that came from a witness to one of the killings. Police pulled over white vans Friday and plastered orange stickers on the back to show the vehicles had been checked. Moose said investigators were chasing more than 500 leads.

Each Maryland victim was felled by a single bullet, apparently from a high-powered rifle or handgun. Police said evidence indicated the killer was some distance away and used .223-caliber bullets.

The search Friday went on amid a mix of fear and defiance among residents of the economically and culturally diverse slice of the suburban Washington county where most of the shootings occurred.

'I can't stay at home'

All over Montgomery County, people did what they usually do on a Friday, but they moved slowly and quietly, glancing at trees, bushes and rooftops. Many said they were afraid but wouldn't stop getting groceries, going to work or leaving their children with a baby sitter.

"I had to shop. I need to eat. I can't stay at home all day," said Kira Leonova, who works at a bookstore near one of the slaying scenes. "I have to work and I have a family."

Dexter Evans, 20, scanned the traffic as he waited for a bus to Rockville, and he took a second look at every white truck. "You can't even walk down the street without looking over your shoulder," he said.

Schools opened with extra police patrols and calls poured into 911 dispatchers about suspicious noises.

The five Maryland victims died within five miles of one another during a 16-hour span Wednesday evening and Thursday morning. All were gunned down in broad daylight in very public places: two at gas stations, one outside a grocery, another outside a post office and the fifth as he mowed the grass at an auto dealership.

The sixth victim, Pascal Charlot, was fatally shot as he stood on a corner in northwest Washington near the Montgomery County line Thursday night, nearly 12 hours after the fifth Maryland shooting.

Authorities said the same rifle was used to kill Charlot, the woman outside a post office and the two people at the gas stations.

"There's still no information to lead us to think our victims are associated," Moose said. "They don't appear to be anyone's enemies, just random targets."

Carin Saez, 27, found herself going back to school Friday to pick up her 12-year-old niece, deciding it was too dangerous. Saez said she would not let her own children go back to school until the killer was caught.

"I was petrified to even go to the store last night," Saez said. "My kids were scared. They didn't even want to go outside. They're more scared now than on Sept. 11."

Officials at Richard Montgomery High School in Rockville canceled a Friday night pep rally and police were posted at the football game against rival Wooten High. Dani Young, a 17-year-old senior, said: "It kind of ruins the mood of homecoming."

In Bethesda, Mary Patterson said as she leaving home for a hair appointment: "I'm not afraid. After all, I'm 81 years old -- my time could be anytime."