Thousands participated Tuesday in a procession to the memorial for four murdered Washington state police officers, with initial estimates putting the crowd of mourners as large as 20,000.

Pierce County officials said 2,000 police and fire vehicles from more than 150 agencies joined the procession to the service at the Tacoma Dome for the victims of a shooting late last month a coffee shop.

The shooter, Maurice Clemmons — who gunned down Mark Renninger, 39, Ronald Owens, 37, Tina Griswold, 40, and Greg Richards, 42 — was killed Dec. 2 in a shootout with a Seattle police officer after a two-day manhunt.

Vehicles left McChord Air Force Base at 10 a.m. Tuesday on a route past the Lakewood Police Department and were joined by Lakewood police and families of the murdered Lakewood officers.

Several candlelight vigils have been held for the officers since the shooting, but Tuesday's memorial service and procession is believed to be the largest in state history.

The three-hour procession route took participants under a large American flag hanging from arching fire truck ladders, and it was joined by local law enforcement in Washington state, along with those traveling from as far away as New York, Chicago, and Boston.

The officers' family and friends served as pallbearers, carrying the the caskets of Renninger, Owens, Griswold and Richards into the Tacoma Dome.

A ceremonial honor guard opened the ceremony, and every officer in the arena stood at attention and saluted as the families of the four officers and members of the Lakeland Police Department were led to their seats.

"They were good people, and they were great cops," Lakewood Police Chief Brett Farrar said at the service. "And they will be truly missed."

In addition to eulogies from family, friends and public officials, mourners watched a video tribute to the officers.

Cpl. Jack Hundial, of Surrey, British Columbia, was one of 1,000 Royal Canadian Mounted Police in attendance. He said he and his colleagues wanted to show their support because "it could have been any of us."

"I think about their families," he said. "I don't think you ever find true closure for something this tragic."

Flags at all state facilities were lowered to half-staff Tuesday, on orders from Gov. Chris Gregoire, who spoke at the service.

Additional space for the public to view telecasts of the service was made available at the University of Washington Tacoma, Pacific Lutheran University and the Christian Faith Center in Federal Way.

LIVESHOTS: Lakewood Officers Remembered

Clemmons allegedly went to the coffee house on the morning of Nov. 29 and opened fire on the Lakewood officers as they caught up on paperwork at the beginning of their shifts. He fled but was wounded by one of the dying victims who shot him in the stomach.

SLIDESHOW: Deadly Lakewood Police Shooting

Four people have been charged with helping Clemmons, 37, avoid capture, and two more indictments are expected.

Police allege the four suspects gave first-aid to Clemmons, helped him change clothes and made arrangements to get him to other locations.

Arkansas Gov. Mike Beebe said Thursday he's willing to look at any recommended changes in how his state sends parolees to Washington state after Clemmons, an Arkansas parolee, shot the Lakewood officers.

Beebe told reporters he spoke for about half an hour with Gregoire and that he's confident Arkansas correction officials properly handled Clemmons' case after he was paroled.

At issue is a warrant that Arkansas officials issued for Clemmons. Arkansas corrections officials say the warrant should have been enough to keep Clemmons in jail on a charge in Pierce County, Wash. But Washington officials say it wasn't handled properly in Arkansas.

The Associated Press contributed to this report.