A gunman who shot dead four US police officers as they sat in a cafe in Washington State will be brought to justice, a police chief has vowed¹.

美国一位警察局长发誓，会将华盛顿州一个咖啡馆杀害四名警官的持枪歹徒缉拿归案。

Police have issued arrest warrants for Maurice Clemmons

Lakewood Police Chief Bret Farrar described the four victims as "good people and great officers".

Earlier, police laid siege（包围，围攻） for several hours to the Seattle house where the chief suspect in the case, named as Maurice Clemmons, was believed to be.

But when officers stormed in, the property turned out to be empty.

Police said warrants for first-degree murder had been issued against Mr Clemmons and the search for him was continuing.

Chief Farrar, his voice breaking with emotion, told a news conference that he had visited the families of the murdered officers.

"It was the hardest thing I have ever done and I hope I never have to do it again," he said.

"We will get through this, although it is a very difficult time for us and the families."

He praised police teams involved in the hunt for the killer² and added: "There is no doubt in my mind that this person will be brought to justice."

The four police officers - Sgt Mark Renninger and officers Ronald Owens, Tina Griswold and Greg Richards - were shot dead at the cafe in Parkland, 40 miles (65km) south of Seattle, on Sunday morning.

On Monday, detectives revealed that the killer had himself been shot during the attack.

"We don't know if he's still alive," Pierce County sheriff's spokesman Ed Troyer told CBS television.

"If he isn't, it's because he succumbed³ to（屈服于，死于） the wound he received yesterday when he was in the struggle with the police officer that managed to get a shot fired at him before he was killed."

Acting⁴ on a tip-off, officers sealed off part of the Leschi area of Seattle on Sunday night, while armed Swat team members ringed the house where Maurice Clemmons was believed to be holed up（冬眠，躲藏）.

As the siege continued through the night, witnesses heard loud bangs, breaking glass, explosions and shots.

But it was later revealed that Mr Clemmons was not in the house.

The Seattle Times reported that police were also searching the University of Washington campus following a reported sighting of Mr Clemmons.

US media said Mr Clemmons has a long criminal history including a recent arrest for allegedly assaulting a police officer in Washington state.