SEATTLE - A gunman alleged to have shot dead four police officers was on the run Monday as shell-shocked officials condemned the killings as a "senseless act of violence."

After laying siege to the home in Seattle's Leschi neighborhood for 11 hours, police SWAT teams entered the building where convicted felon Maurice Clemmons was believed to be holed up.

Pierce County Sheriff's Office spokesman Ed Troyer said, however, that the house was empty and that Clemmons, who was wounded after opening fire on his victims on Sunday, was believed to be armed and still at large.

"We have cleared the residence ... the suspect is not there," Troyer told King 5 local television in Seattle.

Police used robots to search a trailer near the home during the siege early Monday.

Police negotiators, meanwhile, used loudspeakers and airhorns in an effort to communicate with Clemmons, urging him to call 911.

A murder warrant has been issued, while a reward leading to Clemmons's arrest has been increased to 125,000 dollars.

Police earlier confirmed that Clemmons was wounded by return fire after he ambushed four police officers with a handgun in a coffee shop early Sunday near McChord Air Force base in Tacoma, south of Seattle.

All four officers, members of the fledgling Lakewood Police Department founded in 2004, were married with children.

Lakewood city officials and police were still struggling to come to terms with the attack on Monday.

"What happened yesterday was an outrageous, senseless act of violence," Lakewood city manager Andrew Neiditz told reporters.

"Yesterday our hearts were broken but our resolve was not."

"We will get through this," Lakewood police chief Bret Farrar added, his voice wavering with emotion.

"However, it is a tough time for us and the families of our fallen officers."

Troyer earlier said the shootings may have been motivated by general hatred of law enforcement on the part of Clemmons.

Clemmons was only released on bail last week after being held for several months on charges of assaulting a police officer and child rape.

Troyer said it did not appear that Clemmons had a specific reason for targeting the officers involved, saying he was "upset about being incarcerated."

"He was just targeting cops," Troyer said.

Clemmons served only part of a 35-year prison sentence in Arkansas before it was commuted in 2000 by then governor Mike Huckabee, who sought the Republican presidential nomination in 2008.

After his release Clemmons committed two armed robberies and a string of other crimes which earned him another 10 year sentence.

He was later paroled and moved to Washington state.

Huckabee said in a statement Sunday that if Clemmons was responsible for the shootings "it will be the result of a series of failures in the criminal justice system in both Arkansas and Washington State."

However, a victim of a 1989 street robbery involving Clemmons expressed incredulity that he had been released.

"I'm flabbergasted," Karen Hodge, 68, told the Seattle Post Intelligencer. "He should still be in prison."