Local and federal investigators continue to hunt for a sniper responsible for the killing of eight residents and wounding of two others in the Washington area. Authorities have released composite pictures of a truck, described by witnesses at more than one of the shooting sites.

The pictures issued by authorities show a white box truck with writing on the side.

Authorities are also on the look-out for a sleeker mini-van, raising questions about the number of people or vehicles involved in the sniper shootings.

Charles Moose, the police chief in suburban Montgomery County, where the first shootings took place, was asked about that on the ABC program, This Week. "We've tried to be clear, we've not ruled out anything. We've tried to keep an open mind. We want to let the evidence and the facts of the case lead our investigators to bring this to closure," the police chief said.

All of the victims have been hit with a single shot from a high-powered rifle. A reward for information leading to a conviction has grown to a half-million dollars.

Robert Ressler is a former criminal profiler for the U.S. Federal Bureau of Investigation, who has interviewed serial killers.

Appearing on the NBC program Meet the Press, Mr. Ressler says the information is limited. But the evidence suggests the sniper is not acting alone. "He is doing things he has thought out very carefully. It is not precipitous in any way. Everything would indicate to me, with the size of the truck and getting away from the scene the way this individual is doing, it tells me there are two people involved, not just one," he said. Former FBI criminal profiler Clint Van Zandt, also appearing on Meet the Press, said he thinks the sniper is playing off of media accounts. But Mr. Van Zandt said it is unlikely the shooter would stop his killing spree, if there were a media black out.

"I think he would continue to find ways to get our attention, but we just might not be feeding him so much at this time. This guy enjoys the attention. Where(as), many serial killers have some sort of relationship hands-on with their victim, he may have his relationship vis-a-vis the media," Mr. Van Zandt said.

All the victims were shot from a distance as they went about their normal routines, such as cutting grass or walking through parking lots. Several of the shootings have taken place at gasoline stations.