Police near Washington have confirmed that the shooting death of a bus driver in Maryland Tuesday was the work of a serial sniper who has sown fear and anger throughout the metropolitan area.

Conrad Johnson, 35, officially became the 13th victim of a mysterious sniper who has killed ten people and wounded three others in a shooting spree that began three weeks ago.

Police had no new public messages for the sniper on Wednesday. But they did appeal to people who may have witnessed Tuesday's shooting to come forward, especially non-citizens who may be fearful of jeopardizing their immigration status.

"Perhaps some of our immigrant community members feel like there would be some problem for them because of their status or some questions with regards to their status if they come forward," acknowledged Montgomery County, Maryland, Police Chief Charles Moose at a news conference. "So we hope that is not the case. But if that is the case, we want to stress that that is not our interest in this matter. We will work with them. We need to hear from them."

Nervous parents accompanied children to school Wednesday just hours after police read aloud a warning from the sniper that said, "Your children are not safe, anywhere at any time." The warning was contained in a letter that police believe the sniper left behind at last Saturday's shooting in Virginia.

At the White House Wednesday, President George W. Bush expressed sadness about the sniper attacks and said "the full resources of the federal government" are available to local police trying to solve the case, saying he and First Lady Laura Bush "join our fellow Americans in prayer as we pray for the families and friends who have lost loved ones, as we pray for the safety of our fellow citizens, as we pray for the quick end to this period of violence and fear."

Police are also urging the public to continue to call special telephone tip lines with information that may be helpful to investigators.

Unidentified law enforcement officials have told several news organizations that the sniper's letter also included a complaint that he had been unable to get through to police because FBI operators had cut him off.

Investigators acknowledge that public anxiety and anger will continue to grow as long as the sniper remains at large. "We would have liked to have solved this by now, but it is certainly not for the sake that we are trying our best," said Michael Bouchard of the federal bureau of Alcohol, Tobacco and Firearms, the agency that has linked the murders through ballistic evidence. "So, it may take a little longer, but we are certainly confident that we are going to resolve this."

Meanwhile, Maryland's governor says he is considering using National Guard troops to help safeguard polling places from the sniper for congressional and local elections that will be held on November 5.