Lovelle Mixon

Three police officers in Oakland, Calif., were gunned down on Saturday and a fourth was critically wounded in an afternoon of violence that began with a routine traffic stop and ended with a grim 9 p.m. news conference to announce the toll of one of the worst days in the police department’s history.

The police said a 27-year-old gunman shot two motorcycle officers after being pulled over at 1:15 p.m. Saturday, killing one and wounding the other. The gunman then fled to a nearby apartment.

A manhunt ensued through the streets of the East Oakland section of the city. When the police tracked down the gunman and burst into the apartment, he shot two more officers with an assault rifle, killing them both, before he was shot dead by the police.

The two officers who were shot at the scene of the traffic stop were identified as Sgt. Mark Dunakin, 40, and Officer John Hege, 41. The police said Mr. Dunakin was killed and Mr. Hege was in “grave condition” at Highland Hospital on Saturday night.

The two Special Weapons and Tactics officers who were killed in the apartment were identified as Sgt. Ervin Romans, 43, and Sgt. Dan Sakai, 35.

The police said the gunman was Lovelle Mixon, of Oakland, and said he was on parole for assault with a deadly weapon. They had a warrant out for his arrest.

They did not say why officers pulled over Mr. Mixon to begin with, but said it appeared to be a routine traffic stop.

They said he appeared to be the only suspect.

In a somber news conference at police headquarters in Oakland, police officials struggled to describe the enormity of the violence.

“I’ve been to a lot of police funerals,” the acting police chief, Howard A. Jordan. “This will be a pretty traumatic and daunting task for us as a department. It’s not going to be easy and it’s not going to happen tomorrow, but in the end, I think we will move on from this.”

The news conference was attended by most members of the Oakland City Council as well as the former mayor, Jerry Brown, who is now the state’s attorney general.

Gov. Arnold Schwarzenegger of California released a statement Saturday lamenting the deaths. “This is a tragic day for law enforcement officers everywhere,” Mr. Schwarzenegger said. “All four officers dedicated their lives to public safety and selflessly worked to protect the people of Oakland.”

A man who works at a barbershop near the site of the traffic stop told The Oakland Tribune that the two officers had wounds to their faces.

“I went over to one officer and saw he was bleeding from his helmet pretty bad,” the witness said. “The other officer was laying motionless.”

Tension between police and the community has escalated since the fatal shooting of an unarmed 22-year-old, Oscar Grant III, by a transit police officer on New Year’s Day.

Mr. Grant was shot at close range while lying face down on a train platform. He was among several people who had been removed from the train by police officers investigating a fight. The former Bay Area Rapid Transit officer accused in the shooting, Johannes Mehserle, has pleaded not guilty to a murder charge.

Violent protests hit the streets in the weeks after Mr. Grant’s death. On Jan. 7, more than 100 people were arrested after protesters marched through the city breaking store windows and setting cars and trashcans on fire.

Oakland’s black community and law enforcement have had a tense relationship for years, including a corruption case known as the Riders case in which a group of police officers were accused of abusing and falsely accusing suspects. Three of the officers were acquitted, but the case nevertheless damaged the department’s reputation.

The Associated Press reported Saturday that people lingered at the scene of the traffic-stop shooting. About 20 bystanders taunted the police.