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 14th 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 over 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 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 on condition of anonymity said the note demanded $10 million.

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."