ROCKVILLE, Md. (AP) - Hours after a bus driver was killed in a shooting that appeared to be the work of the Washington-area sniper, police said Tuesday they received a message warning: "Your children are not safe anywhere at any time."

The chilling warning apparently was discovered by police Saturday night at the scene of a shooting outside a Virginia steakhouse. Police said in came in the form of a "postscript."

Montgomery County Police Chief Charles Moose also said police "will be responding soon" to the latest message and did not take any questions from the media.

On Monday, police asked the shooter to call them, saying investigators had received a message too muddled to understand.

Early Tuesday, bus driver Conrad Johnson, 35, was fatally wounded as he prepared for his morning bus route in nearby Aspen Hill, the suburban Washington community where the shootings began Oct. 2. In all, 12 people have been shot by the sniper in Maryland, Virginia and Washington, D.C.; three were critically wounded.

Johnson, a father of two, was shot as he stood at the top of the steps of the bus shortly before 6 a.m.

Moose has now used four briefings to communicate directly with the person believed to be the sniper.

A senior law enforcement official speaking to The Associated Press on condition of anonymity said Tuesday a letter found near the scene of Saturday night's sniper shooting in Virginia suggested the killer wants several million dollars.

Two other law enforcement sources told the AP the letter also contained information that police interpreted to be a vague threat to children. One of the sources, who also spoke on condition of anonymity, said the letter was "lengthy."

Schools in the Richmond area, near Saturday's shooting, remained closed a second day Tuesday.

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.

The shooting happened near an apartment building and 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. He didn't know if anyone else was on the bus.

Agents for the federal Bureau of Alcohol, Tobacco and Firearms combed the crime scene. A police dog searched near a basketball court in a park, and police helicopters flew over the scene.