State Department of Corrections officials identified the gunman as Isaac Zamora, 28, who had just served a six-month jail sentence for drug possession in Skagit County. He surrendered to authorities after Tuesday's deadly spree.
A bullet hole was found in the passenger side window of the suspect's truck, reports CBS affiliate KIRO-TV in Seattle.
Zamora, had been released Aug. 6 and was under community supervision by Corrections officers, spokesman Chad Lewis said. Zamora's mother said she had tried repeatedly to get help for her son, whom she described as "desperately mentally ill."
"We're so devastated for the families," Dennise Zamora told The Associated Press by telephone. "I wish it would have been him or me that was killed. That's how deeply I feel about it."
Six people died Tuesday, including a Skagit County sheriff's deputy who had tried to help the suspect's family in the past, and a motorist who died on Interstate 5 as the suspect fled.
At least two others were wounded, including a Washington State Patrol trooper shot while trying to stop the shooter on the freeway, the State Patrol said. A motorcyclist also was shot in the arm at a Shell gas station.
"There are a lot of answers that are just not available right now," Everett police Sgt. Robert Goetz told a news conference late Tuesday night.
The slain deputy was identified as Anne Jackson, 40, whom Mrs. Zamora described as a sympathetic figure who had tried to help the family in the past.
"She was very gracious," Dennise Zamora said. "She knew exactly what we were going through, said her brother was going through some similar stuff."
The dead were found at multiple crime scenes. They included Deputy Jackson, shot while responding to the initial call by Dennise Zamora, and a second person killed at the same location near the small town of Alger; two construction workers found shot nearby, and a body found a few houses away, Trooper Keith Leary said. The civilian motorist was killed and the trooper was injured along I-5 near a rest stop, Leary said.
The wounded trooper, grazed on the arm by a bullet, drove himself to a nearby hospital for treatment and was later released. He was identified as Trooper Troy Giddings.
The other victims were not immediately identified.
Since his release, Zamora had reported as ordered to Corrections officials and passed drug and alcohol screenings, Corrections Secretary Eldon Vail said.
"I want to extend our profound sorrow and heartfelt sympathy for the victims of the shootings," Vail said. "This is a senseless loss of the lives of innocent citizens and a member of our law-enforcement family."
Gov. Chris Gregoire called for an independent third-party review to be led by the head of the Washington Association of Sheriffs and Police Chiefs and a prosecutor to be appointed by the Washington Association of Prosecuting Attorneys.
"I will be directing them to look into how this case was handled and provide an initial report to me within 10 days," Gregoire said late Tuesday night.
After the shootings in the Alger area, the armed man raced south on the freeway at speeds in excess of 90 mph, with troopers, sheriff's deputies and Mount Vernon police in pursuit, Leary said.
The first shootings were reported shortly after 2 p.m. Tuesday and the suspect was in custody by about 4:30 p.m. He turned himself in at the Skagit County Sheriff's Office.
State Patrol troopers temporarily closed all southbound lanes of I-5 north of Burlington for a period Tuesday evening, backing up traffic for miles, as they investigated that crime scene.
Hours after the shootings, a group of residents gathered at the Alger Bar and Grill to watch the television news.
"It's devastating for this town," Steve Thomas said, "The people here are very law enforcement-friendly. It's very somber."