ROCKVILLE, Md. -The shooting of a bus driver outside Washington has been linked to others attributed to the sniper who has stalked the region for three weeks, investigators said Wednesday.

Ballistic as well as other evidence has linked Tuesday’s shooting of Conrad Johnson, 35, said Michael Bouchard, an agent with the federal bureau of Alcohol, Tobacco and Firearms.

Anxious parents, meanwhile, took their children back to schools under tight security Wednesday after police revealed a chilling warning _ “Your children are not safe, anywhere at any time” - apparently written by the sniper.

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

Bouchard reminded the public Wednesday that “we’re all parents, and certainly concerned about safety of our kids as well as co-workers.”

One of the sniper’s victims was a 13-year-old boy shot outside his Bowie, Md., school who remains hospitalized in serious condition.

In response to reporters’ question, Bouchard defended the performance of the multi-agency task force.

“All the leads that come in, we follow those every day and eliminate potential suspects. Progress is made every day and puts us one step closer,” Bouchard said.

He also said investigators have not withheld information vital to the public. But if information is made public “before we were ready for it to go out, it inhibits our ability to do the job we need to be doing.”