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. Last Updated: 07/27/2016

U.S. Sniper Targets Children

ROCKVILLE, Maryland -- Anxious parents took their children back to schools under tight security Wednesday after a chilling warning -- "Your children are not safe, anywhere at any time" -- apparently written by the sniper who has shot at least a dozen people in the greater Washington area.

The words were left at the scene of a shooting Saturday night in Ashland, Virginia. Montgomery County Police Chief Charles Moose, leader of the multi-agency task force investigating the shootings, revealed them Tuesday, the day bus driver Conrad Johnson was shot to death in Aspen Hill, Maryland, while preparing to begin his morning route.

Authorities suspect Johnson, 35, is the 13th victim of what may be a multimillion-dollar extortion attempt, although confirmation awaits tests on the bullet that killed him. The note demanded $10 million, said a senior law enforcement official, speaking on condition of anonymity.

Moose urged the killer to continue a dialogue that began after Saturday's shooting.

"It is important that we do this without anyone else getting hurt," Moose told reporters Tuesday evening in comments directed at 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," Moose said.

"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. Moose offered to set up a private post office box "or another secure method" if the killer preferred.

"You indicated that this is about more than violence," Moose said. He did not elaborate.

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

The letter writer also called police inept and described six unsuccessful attempts to reach investigators by telephone since the attacks began, complaining that operators hung up on the calls, The Washington Post reported, citing sources it did not identify.