Police revealed that a message apparently from the Beltway Sniper found on Saturday night included a chilling warning that threatens the safety of children.

Hours after the sniper appeared to have struck again in the shooting death of a bus driver Tuesday, Montgomery County police chief Charles Moose announced previous communication from the gunman included the warning: "Your children are not safe anywhere at any time."

Moose later issued a new plea to the killer, urging the gunman to continue a dialogue with investigators.

"It is important that we do this without anyone else getting hurt," said Moose, who had publicly pleaded for more communication in the two days before Tuesday's slaying.

In an evening news briefing, the police chief suggested authorities had repeatedly been in contact with the sniper.

"We have researched the options you stated and found that it is not possible electronically to comply in the manner that you requested," he said. "However, we remain open and ready to talk to you about the options you have mentioned."

He said the sniper is seeking an 800-telephone number to talk with authorities, and he offered to set up a private post office box "or another secure method."

"You indicated that this is about more than violence," Moose said. "We are waiting to hear from you."

He refused to take questions from reporters.

The bus driver was gunned down in Aspen Hill, the same community where the shootings began Oct. 2. If the latest attack is confirmed, the sniper will have killed 10 people and critically wounded three others in Maryland, Virginia and Washington.

Conrad Johnson, 35, was fatally wounded just before 6 a.m. as he stood on the top step of his bus, setting off a police dragnet and snarling traffic in the suburbs north of the nation's capital. He died later at a hospital.

Earlier, Moose suggested police had received a new message from the killer, believed to be the fourth during the investigation.

The warning about children's safety was discovered by police outside a Ponderosa steakhouse just north of Richmond, Va., where the sniper critically wounded a man Saturday night.

Moose said the warning came in the form of a "postscript," but refused to describe the rest of the note.

However, a senior law enforcement official speaking to The Associated Press on condition of anonymity said the note demanded $10 million.

More than 140,000 students in the Richmond area remained home Tuesday as schools were closed for a second day. School officials cited information from police in shutting down, prompting questions for Moose, whose office is leading the sprawling investigation.

Moose said investigators recognized "the concerns of the community" and decided to provide the "exact language that pertains to the threat."

The Virginia schools will reopen Wednesday under heightened security. As of Monday evening, no Maryland schools had decided to close.

In Maryland, Kathy Franco, who was shopping with her year-old son, Liam, and six-week-old daughter, Katherine, was angered by the warning about children.

"As a parent, it just completely brings out every animal instinct," she said. "These two are the most important things in the world for me."

Immediately after Tuesday's shooting, police put a widespread dragnet into place, clogging traffic on Connecticut Avenue, one of the main arteries into Washington, just as the morning commute began.

But police came up empty, and Moose said there was no suspect or vehicle description to report.

"We have not been able to assure that anyone, any age, any gender, any race — we've not been able to assure anyone their safety," Moose said.

The shooting happened near a wooded area along Connecticut Avenue. The bus was parked at a staging area where drivers get ready for their morning runs, state police spokesman Cpl. Rob Moroney said.

Police refused to say whether anyone else was on the bus.

All the confirmed sniper victims were felled by a single shot. Several residents of a neighboring apartment complex reported hearing one loud bang Tuesday morning.

"It wasn't a pop like a handgun. If it was a gun, it was a high-powered weapon," said Tim Roberts, a carpenter who lives nearby. He said he knew about the sound of weapons from his military service.

Johnson, a married father of two and a 10-year county employee, was pronounced dead at Suburban Hospital in Bethesda. In his Oxon Hill neighborhood of townhouses and tree-lined streets, residents expressed sorrow at his slaying.

"This is hitting hard. This not only strikes us as a community, but as a people, as a nation," said Harold McClam, who frequently saw Johnson leaving for work in the mornings. "When they catch this guy they need to punish him to the full extent of the law."

Fellow bus driver Wade Vassell said a friend had called him with the bad news.

"I know my boy eight years. He was my friend," Vassell said. "I'm nervous, real nervous."

The Associated Press contributed to this report.