After a year when efforts to reform the Prince George's County Police Department appeared to be taking hold, a spate of shootings by officers this week has renewed concerns that arose during the decade when the agency led the nation in fatal shootings.

In a three-day period, county police shot an unarmed man in the back, shot a motorist by accident, wounded an unarmed passenger and killed a mentally unstable man. Before that, it had been seven months since a police officer shot someone and a year since the last fatal shooting.

Although Prince George's officers have received intensive training in the use of nonlethal methods to subdue people and are subjected to greater scrutiny when they use deadly force, this week's shootings represent a recurrence of patterns that have long hampered the department. From 1990 to 2000, Prince George's police fatally shot more people per officer than any other major police force in the nation.

On Thursday, Cpl. Etienne Jones shot and wounded an unarmed driver, 33-year-old Mohammad S. Mohammad, in Adelphi after police mistakenly assumed the man was driving a stolen car. Although police commanders called the shooting an accident, it fit the agency's profile during the 1990s, when almost half the people shot by police were unarmed and many had committed no crime.

On Wednesday, Cpl. Charles K. Ramseur also shot an unarmed man, wounding him in the back, for reasons that remain unclear. It was the fourth shooting of Ramseur's career. About 20 percent of Prince George's officers who have been involved in shootings since 1990 had shot someone before.

Late Thursday, Sgt. Henry Norris and Officer Felipe Ordono fired into a sport-utility vehicle in Hyattsville after the driver allegedly tried to run over a detective. One of their bullets struck an unarmed passenger. The driver was not hit. The SUV later was determined to have been stolen.

Most big-city police departments forbid their officers to fire at cars, calling it a risky practice that endangers bystanders. In Prince George's, however, the practice is not banned, and officers have wounded or killed 19 people in cars in the past decade.

Police officials were at a loss yesterday to explain the sudden spate of shootings, calling the incidents completely unrelated.

Police Chief Gerald M. Wilson, while defending his officers, sought to reassure the public that he would not tolerate excessive force.

"I have not and will not condone excessive force," Wilson said. "But I cannot ask my officers to go out there and put themselves in harm's way without giving them the authority to defend themselves and the lives of others."

Wilson met yesterday with County Executive Jack B. Johnson, who said he expressed to Wilson "deep concern about the shootings, that there were so many in such a short amount of time."

Johnson (D) was elected last month after he made police reform a centerpiece of his campaign, and he invited reporters to his office yesterday to underscore his concern. His predecessor, Wayne K. Curry (D), rarely commented on police shootings or police brutality cases.

"My role as the manager of this police department now is to bring about certain changes so that when we have these shootings, you won't have all these questions," Johnson said. "And I know that at the end of the day, we will do that. We will get the job done."

Glenn Ivey, the newly elected Prince George's state's attorney, said he would probably commence a grand jury investigation into each of the shootings next week. He said the grand jury probes will vary in length.

"For some shootings, we've got a lot of civilian witnesses," he said. "For others, we've only got a couple of witnesses."

Ivey said his office sent an independent investigator to the scene of each police shooting. That represents a change in policy from Johnson's tenure as state's attorney, when prosecutors relied largely on the police department to probe its own shootings.

Wilson withheld judgment on his officers' actions, citing the pending investigations, but indicated that the week's first shooting appeared to be justified.

In that case, Sgt. Russell Watson killed Eric Stewart, a 31-year-old Landover Hills man who police said was holding a knife to his mother's throat and using her as a shield. A police spokesman said a supervisor ordered Watson to shoot.

Police said that Stewart appeared unstable and that his family had called them in the past to report strange behavior on his part, including a threat to kill his sister.

The case, however, illustrated a persistent problem for the department: dealing with mentally ill suspects. Police have shot at least 13 mentally or emotionally disturbed people since 1990. In many cases, officers initially were called to help the people they ended up shooting.

Police commanders said they were examining the shooting late Wednesday in Capitol Heights, when Ramseur opened fire on 22-year-old Desmond Eugene Ray after he stepped out of a car.

A police spokesman said Ramseur shot Ray once in the back after he "perceived a threat," but the spokesman declined to say what that threat was.

Ray was taken in critical condition to Washington Hospital Center, where he remained last night. Police said they found drugs and a handgun inside his car, though it was unclear whether the items belonged to Ray.

Police have refused to provide other details of the shooting, Ramseur's fourth since joining the department 14 years ago. In July 1999, he was transferred from the Emergency Services Team, according to court records, because police said he "showed a lack of responsibility in his handling of weapons." Ramseur sued the department and was reinstated.

Redmond Barnes, a member of the People's Coalition for Police Accountability, a watchdog group, said the department needs to take a closer look at officers who have a history of shootings.

"You'll never get officers to behave differently if you don't punish or discipline them," he said. "What does it take to discipline an officer, if you can't discipline an officer who has been involved in several questionable shootings?"

Geoffrey Alpert, a criminologist at the University of South Carolina, said it's "extremely rare" to have so many police shootings in such a short period.

"It's almost a statistical phenomenon," he said. "It seems to be an awful stroke of bad luck."

Alpert, who has studied the use of lethal force by police in Prince George's and the District, said some of the cases warrant close scrutiny.

"So you have one that's probably totally legitimate," he said, in reference to the mentally ill man armed with a knife. "[But there's] another you don't know much about except that someone was shot in the back, another that appears to be mistaken identity, and a fourth at a moving vehicle, which I always have problems with."

Last year, Prince George's police shot seven people, two of whom died.

Wilson, the police chief, acknowledged that the rush of police gunfire this week has left him rattled, but he added: "Our record is intact. It is a low year, relative to last year."

Staff writers Ruben Castaneda and Paul Schwartzman contributed to this report.