Updated

The total reward for information leading to the arrest of the serial sniper terrorizing the Washington, D.C., area reached $237,000 Tuesday, as Maryland Gov. Parris Glendening said the shooter is nothing more than a coward.

Glendening made an emotional appeal for the killer to turn himself in for the shootings that have so far left six people dead and two injured.

"This is a person who is shooting elderly men, shooting women, and now shooting little children," Glendening said, one day after a 13-year-old boy was critically wounded as he walked to school. "And I really think if there is any message, it is for this individual to turn himself in, to stop this insane killing."

The governor said the shootings were the acts of an "absolute coward."

"No one is looking up to him," Glendening said. "No one is thinking this is a great act he is doing."

In an afternoon briefing, Montgomery County Executive Douglas Duncan announced that the reward for information leading to the killer's arrest had reached $237,000.

That included $50,000 from the county’s general fund and $100,000 in state money pledged by Glendening.

Earlier in the day, a Montana businessman named Tim Blixseth added $50,000 of his own, spurring Gov. Judy Martz to send letters to the governors of all 50 states asking them to urge corporate America to contribute to the fund.

"After the events of Sept. 11, we have seen a shift in the way that Americans step forward to help one another," Martz said. "The attacks that are occurring in Maryland could as easily be occurring in Montana, Michigan, Wyoming, or South Carolina. I ask that all governors work in their states to support the efforts in Maryland to catch this killer."

Back in Maryland, anxious parents accompanied their children to school or kept them at home Tuesday, one day after the sniper linked to the murder of six adults critically wounded a middle-school pupil.

Security firms across the region reported a surge in interest. About 50 Starbucks stores removed their outside seats. And mental health counselors scrambled to set up crisis hot lines for people upset by the string of shootings that have left six people dead and two wounded since last week.

The latest victim, a 13-year-old boy, remained in critical but stable condition Tuesday with a wound to the chest. He was shot early Monday after he was dropped off at school.

Investigators were examining a shell casing found outside the school to see if it was linked to the shootings.

With few other solid clues or witness accounts, a task force of federal, state and local investigators is sifting through more than 7,500 phoned-in tips. Police said they have 1,400 credible leads but refused to disclose details.

"I can't stop going to work, the children can't stop going to school," said Henry Ollie, 48, leading his 12-year-old son, Charles, to the front door of Benjamin Tasker Middle School in Bowie, where the latest shooting happened. Ordinarily, Charles takes the bus.

Some parents served as volunteer guards, watching over intersections. But it appeared many decided to keep their children home as Monday's shooting fueled heightened anxiety for families in already nervous suburbs. Some buses arriving at schools carried fewer students than usual.

Prince George's County, the scene of the latest shooting, sent two helicopters to patrol the area and had police officers at every school. Police in neighboring Montgomery County, where five people were killed last week, also guarded schools.

"As a community we clearly remain anxious," Montgomery County Police Chief Charles Moose said Tuesday morning. "We have a very visible patrol in and around our schools, a very visible patrol around our freeways."

There was no immediate comment from Prince George's officials on precise attendance figures, but Jerry Weast, the Montgomery school superintendent, said attendance there was "at the lower end of the normal range."

Police in Prince George's County served a search warrant during the night, but the police chief said no arrests were made. No further details about the warrant were offered.

"All of us will have to fight," Prince George's County Police Chief Gerald Wilson said Tuesday morning. "We cannot allow one individual to shut that spirit down in us."

A task force including local and state police, the FBI and the Secret Service mobilized to pursue the sniper, but police acknowledged having few clues or eyewitness accounts to solve one of the most frightening serial killings in memory.

The sniper has shot eight people since Wednesday, killing six; police have discerned no pattern among the victims. One died on a Washington street, the others within five miles of each other in Montgomery County.

Authorities said they were looking at a number of earlier shootings for possible links to the sniper. Among the cases is a Sept. 14 shooting outside a liquor store in Montgomery County that wounded a store employee.

Investigators said tests on bullet fragments taken from the employee were inconclusive, but they still have not ruled out a link to the other shootings.

Arnie Zelkovitz, the owner of Hillandale Beer and Wine, said police interviewed him about the shooting. His 22-year-old employee was shot in the back.

Zelkovitz said he believes his employee was a victim of the sniper: "It just seems too coincidental."

The latest attack Monday morning was 20 miles farther east, in neighboring Prince George's County north of Washington.

Ballistics tests found that the bullet that struck the boy was identical to those that killed some of the others and wounded a woman in Virginia. That woman was released from the hospital Tuesday.

All victims were hit by a single bullet fired from a distance. Police have spoken of a single sniper, but have not ruled out the possibility that more than one person is involved.

President Bush denounced the attacks as "cowardly and senseless acts of violence" and ordered FBI profiling experts and ballistics analysts from ATF to assist local police.

Police and federal agents began to use a geographic profile submitted by investigators that uses crime locations to determine where the killer feels comfortable traveling and may live.

All the victims were shot in public places: the boy outside school, two at gas stations, two in parking lots, another outside a post office, another as he mowed the grass and the eighth on a street corner.

The Associated Press contributed to this report.