A bullet fired by an unseen gunman killed a Montgomery County bus driver shortly before dawn yesterday as he stood in the lighted doorway of his empty bus, while police renewed their fitful, anxious effort to communicate with a murderous sniper who has eluded a manhunt for three weeks.

If yesterday's shooting is linked to the Washington area sniper by ballistics tests, the slaying of Conrad E. Johnson, 35, which occurred in Aspen Hill, would be the 10th killing and 13th shooting attributed to the gunman. And it would mark the return of the mysterious assailant to Montgomery. Five victims were shot to death in or near Aspen Hill on Oct. 2 and Oct. 3 before the sniper attacks spread as far south as the Richmond area, where a man was wounded Saturday.

Despite a massive search for the sniper and extraordinary attempts by authorities in recent days to communicate with the gunman, he remained on the loose after yesterday's shooting as Montgomery Police Chief Charles A. Moose urged the public to be "cautious."

"We remain concerned about the safety of all people in our region," Moose said at a news briefing. "We realize that the person or the people involved in this have shown a clear willingness and ability to kill people of all ages, all races, all genders, all professions, different times, different days in different locations."

Moose declined to reveal most of the contents of a letter that law enforcement sources said was found near the scene of Saturday's shooting and that they strongly believe is an authentic message from the sniper. But the chief quoted the letter's dire postscript: "Your children are not safe anywhere at any time."

Moose also publicly issued yet another in a series of cryptic messages to the gunman. Law enforcement sources said the chief was responding to a communication received earlier in the day from the sniper, who the sources said has demanded $10 million.

"We have researched the option you stated and found that it is not possible electronically to comply in the manner that you requested," Moose said in a statement that authorities said would be understood by the sniper. "It is important that we do this without anyone else getting hurt."

Although authorities as of last night had not publicly linked yesterday's slaying to the sniper, it occurred less than a mile from where the shootings started three weeks ago. The attacks later spread to the District, Prince George's County, and Prince William, Spotsylvania, Fairfax and Hanover counties in Virginia.

Yesterday's gunman struck in the early-morning darkness and was able to vanish before police clamped down with yet another vast dragnet that came up empty.

In what has become a familiar scene across the region, the shooting and subsequent massive police response brought traffic in Montgomery to a standstill, closed some private schools, and fueled the growing sense of fear and anger among the public, as well as frustration among authorities.

Traffic was jammed across three Maryland counties, parts of the District, and Northern Virginia. The American Legion Bridge was closed for several hours in both directions, beginning at 6:10 a.m., bringing the morning rush in the area to a halt.

Moose, who has been speaking publicly for a law enforcement task force searching for the sniper, issued a subdued warning to the public after the dragnet failed to snare a suspect. "We have not been able to assure anyone, any age, any gender, any race -- we've not been able to assure anyone their safety in regards to this situation."

Moose's attempts to communicate with the sniper through the news media began after investigators found the threatening letter, which was tacked to a tree near a Ponderosa restaurant in Ashland, Va., according to law enforcement sources. The discovery came after the sniper's 12th victim, a 37-year-old man from Melbourne, Fla., was shot outside the restaurant Saturday night. He remained hospitalized in critical condition yesterday.

Sources who have read the message said it demanded that authorities put $10 million in a bank account for the letter writer.

One source said the letter berated police as inept and detailed at least six failed attempts by the self-professed sniper to reach investigators by telephone since the attacks began Oct. 2. The letter said those attempts to communicate were not treated seriously by call takers, according to sources.

The letter writer also included a telephone number and said he would be calling police on it at a specified time, they said. That time passed before police finished examining the letter and addressing a problem with the phone number, and no call was received, sources said.

That set in motion a series of bizarre communications between the gunman and police, with Moose talking to the sniper in obscure language via TV news cameras.

Angela Dieckmann, who lives about a block from the site of yesterday's shooting, voiced fears felt by many in the Washington region.

"Can't they just find the guy?" she asked. "It's fascinating that he has so much control. One human being. It is incredible."

The shooting came as Johnson, a driver for the county's Ride On system, was readying his bus for its morning run at a staging area in the 14100 block of Grand Pre Road, near Connecticut Avenue, police said.

Johnson, whose father also had been a county bus driver, was married and had two children. He was struck in the abdomen by a single bullet and died at Suburban Hospital in Bethesda.

James W. Robey, a trauma surgeon who participated in the effort to save Johnson's life, said that in more than two hours of surgery and "heroic maneuvers," he and surgeon Dany Westerband "couldn't stop the bleeding."

"It was the worst liver injury I've ever seen," Robey said. "The injuries were just so extensive."

Westerband was the surgeon on emergency duty when Johnson arrived shortly after 7 a.m., Robey said. Robey arrived soon afterward to do a small operation on another patient but canceled that procedure to help treat Johnson, he said.

Robey said the entry wound was "in front, right under the rib cage -- a small, tiny little bullet hole. There was no exit wound." He said the bullet went "right through the middle of the liver, where the great [blood] vessels are."

Robey also said he and Westerband found "a piece of the bullet," which was turned over to police.

The blue and white bus, with the big numbers 5705 painted on its roof, did not have security cameras on board, officials said. Ride On has wired about 10 percent of its 240-bus fleet with cameras as part of a pilot program.

Johnson was remembered yesterday as a loyal colleague and devoted family man. "He was just as good a human being as you could have the privilege of knowing," said Gino Renne, president of the union in which Johnson and his father were active, Local 1994 of the United Food and Commercial Workers International Union.

After Johnson's death was announced about noon yesterday, police seemed especially dismayed, but they continued to voice confidence that the case would eventually be solved.

But they offered no description of the suspect or suspects; provided no indication that they had any witnesses to the shooting; and offered no details, as they have in the past, of any kind of getaway car.

Moose offered few details of the investigation that today enters its fourth week with little to show except the mistaken seizure Monday of two illegal immigrants at gas stations in Richmond and the arrest of a Virginia man for allegedly giving a false police report.

"We certainly encourage everyone to remain cautious, vigilant about their activities, to remain observant to anything out of place," Moose said.

The sniper's possible return to Montgomery, after four attacks in Virginia, upset residents -- some of whom awoke anew to the sounds of sirens and helicopters -- and paralyzed major thoroughfares north and west of the District.

"I figured he lives in our back yard," said Dick Hottel, a heating and air-conditioning company owner who lives in Rockville, not far from Aspen Hill.

"It's a frightening thing . . . a rattling thing," said Jeffrey Liss, 46, a furniture dealer in Rockville.

Maunette Minor, 39, who lives in an apartment complex near the scene of the shooting, said: "It's so creepy. You never imagine it to be in your back yard. I've always felt so safe in Montgomery County. It makes me not know what the heck to do."

Thousands of motorists and dozens of school buses were trapped in traffic jams yesterday morning as police stopped vehicles and searched cars in an attempt to capture the killer.

Investigators tried moving checkpoints from place to place and asked traffic reporters not to broadcast their locations -- a request with which most complied. "Traffic is standing still, and we can't tell you where it's standing still," said Lisa Baden, a news radio traffic reporter.

Motorists seemed resigned. Many sat with their children buckled in car seats as they waited for their cars to be checked.

Although Montgomery public schools remained open yesterday, several local private high schools and elementary schools closed.

Last night, at an interfaith prayer service in Rockville, Cardinal Theodore E. McCarrick, of the Catholic Archdiocese of Washington, said:

"Join us in prayer for an end to these senseless killings and an end to all the violence that pervades this community. . . . Pray that this terrible thing might end and end quickly."

Billy Kuhn, 8, of Rockville takes part in an interfaith prayer service for the sniper's victims. It was held last night at Faith United Methodist Church in Rockville.Family and friends of Conrad E. Johnson comfort his mother, Sonia Wills, outside Suburban Hospital. With them is a Montgomery police officer.Police and federal agents inspect Johnson's bus at the scene of the shooting, at a Ride On staging area in the 14100 block of Grand Pre Road, near Connecticut Avenue.Specialists from the Bureau of Alcohol, Tobacco and Firearms use special equipment to probe the shooting in Aspen Hill. Conrad E. Johnson, who was struck in the abdomen by a single bullet, was preparing his bus, at left, for its daily run.A helicopter hovers over the scene. The investigation, which involved officers in the air and on streets, brought the morning commute to a halt across much of the Washington area.