Rockville, Md. ? A bus driver was shot to death Tuesday as he was about to set out on his morning route in what authorities fear was the 13th attack by the Washington-area sniper. Police also revealed a chilling warning found at a weekend shooting scene: “Your children are not safe anywhere at any time.”

Montgomery County Police Chief Charles Moose issued a new plea, urging the gunman to continue a dialogue with investigators.

“It is important that we do this without anyone else getting hurt,” he said.

In an extraordinary statement that appeared to confirm authorities are in a desperate parley to stop the killing, the police chief said the sniper had tried to communicate with police in the past several days and he suggested the gunman had made demands.

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

He said the sniper was 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,” said Moose, who had asked the sniper to contact authorities for two days before Tuesday’s slaying. “We are waiting to hear from you.”

He refused to take questions from reporters.

The sniper has killed nine people and critically wounded three others in Maryland, Virginia and Washington since Oct. 2. Authorities were awaiting ballistics tests to confirm that bus driver Conrad Johnson is the killer’s latest victim.

Johnson, 35, was gunned down in Aspen Hill, the same community in suburban Washington where the attacks began.

He was shot once in the abdomen 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. Johnson, a married father of two children, died later at a hospital.

The warning about children’s safety was discovered by police outside a steakhouse near 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. It was unclear whether the demand was linked to the options outlined by Moose.

Kathy Franco, who was shopping Tuesday at a Silver Spring mall with her year-old son, Liam, and 6-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.”

Schools in the Richmond area remained closed for a second day Tuesday, idling more than 140,000 students. School officials had 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 today under heightened security. As of Monday evening, no Maryland schools had decided to close.

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 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.

“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.”