Police near Washington, D.C., have now linked the fatal shooting in the Virginia suburbs late Wednesday to the series of sniper attacks that had left six people dead and two wounded in the past week. The shootings have spread fear and apprehension throughout the Washington area.

The investigation has been frustrating for police who have amassed huge numbers of officers in the search for the killer.

As they process hundreds of tips from the public, authorities say they have no firm idea who is behind the shootings or why they are being carried out.

Charles Moose is the police chief in Montgomery County, Maryland, the area just north of Washington where many of the shootings took place last week. "We continue to keep an open mind with regards to motive," he said. "People are working. Nothing has been ruled out and we want to be open-minded in the continuation of this investigation."

On Monday, a 13-year-old boy was shot and critically wounded outside a middle school in Maryland, prompting local officials to cancel outdoor recess and request that parents accompany their children to school.

The sniper shootings have shaken the entire Washington metro area. The victims, apparently selected at random, have all been struck down from a distance as they went about their normal routines: pumping gas, cutting grass or walking through parking lots.

Maryland Governor Parris Glendening tried to reassure the public on NBC's Today program. "Police are doing everything possible," said Governor Glendening. "They are really aggressively devoting an immense amount of resources. We want people to be careful, we want them to watch, especially, their children, report anything suspicious and use common sense. But at the same time, we must get on with our life. The schools must stay open, the economy must continue, people must do what they are doing."

For the most part, it appears people are trying to go about their daily business, even if they are looking over their shoulder more than they used to. These two Maryland residents say they are constantly aware of the shootings.

WOMAN: "I am really afraid. This is happening right in our neighborhoods and it is just random people, walking down the street, going to the grocery store, going to get some gas. I do those things every day, so I am afraid."

MAN: "It is scary for us. We work in this area. We do not want to be standing outside, quite frankly."

Police have linked the sniper to several shootings because of similar bullet fragments, but are saying very little about evidence. A tarot card was found near the school where the 13-year-old boy was shot on Monday. Written on the card were the words, "Dear policeman, I am God."

Crime experts note that some previous serial killers left some sort of calling card near their victims.

James Fox, a criminologist at Northeastern University in Massachusetts, was asked to speculate about the mental state of the sniper on NBC's Today program. "He wants to call the shots, not just the gunshots, but also how the investigation will proceed," he said. "This is a man who wants to be in control, in command and he commands respect. In fact, the way he addressed the police, 'Dear Policeman', shows respect. Respect is an important theme in his life. Perhaps he does not get it at home, perhaps he does not get it at work, but he is trying to get it through this killing spree."

The shootings have alarmed a community already on edge because of last year's terrorist attack on the Pentagon and the anthrax by mail attacks, a case authorities have yet to solve.