ALAMO -- A California Highway Patrol officer is on life support and his assailant dead following a traffic stop on crowded Interstate 680 Tuesday morning that became violent as hundreds of commuters drove past.

As traffic eased south on Interstate 680 in Alamo at 8:20 a.m., Officer Kenyon Youngstrom was shot through the neck after pulling over a Jeep Wrangler on the shoulder of the freeway, police said. Moments later, a second CHP officer who had pulled up behind the jeep shot the man in his vehicle and aided the injured officer until emergency medical crews arrived, said Contra Costa sheriff's spokesman Jimmy Lee.

Youngstrom, a 37-year-old married father of four, was taken to John Muir Medical Center and was listed in critical condition as of Tuesday night with what KGO-TV described as a severed spinal cord.

Flowers were placed outside the two-story home of the officer in Cordelia, near Fairfield, as CHP colleagues came by to support the family. A man who answered the door at his home declined to be interviewed.

The officer's family "asked for everyone's prayers as Officer Youngstrom fights for his life," Paul Fontana, assistant chief of the CHP, said in a brief statement Tuesday night.

Youngstrom graduated from the academy in 2005 and worked mostly in the Martinez regional office, having been assigned to the Contra Costa office immediately after graduation. Before joining the CHP, Youngstrom served in the U.S. Army Reserve, Fontana said.

The assailant, who later died at the same hospital, was not identified, pending family notification.

The investigation shut down the major East Bay artery, snarling traffic on highways and neighborhood streets throughout Central Contra Costa for most of the day.

The shooting was the first serious line-of-duty incident for Bay Area CHP officers since 32-year-old John P. Miller died after slamming into a tree while pursuing a suspected drunken driver in Livermore on Nov. 16, 2007. The last CHP officer shot in the Bay Area was on Jan. 20, 2010, when a robbery suspect wounded the officer at an Oakland Walgreens. The last fatal CHP shooting was Feb. 17, 2006, in Stanislaus County.

Outside John Muir Medical Center in Walnut Creek, dozens of CHP and police officers walked forlornly into the lobby to hold a vigil. Youngstrom had a large extended family, the CHP said, and relatives from Southern California arrived at the hospital throughout the day. Through Fontana, the family thanked their extended family and friends, the law enforcement and public safety communities, and the public for their support.

"We are family, and the community is part of that family as well," said an emotional CHP Sgt. Diana McDermott. "This is a situation where we all take an oath not to swerve from the path of duty. As you can imagine, it's difficult.

"You think about how tomorrow it may be my day or anyone else's," she said. " ... That's true every day you send a mother, father, husband or wife out to represent and protect the rest of us. We're grateful for all the support we know we're getting from the communities we serve."

Police said the shooting happened around 8:20 a.m. near the Livorna Road offramp from southbound Interstate 680 when two patrol cars pulled the jeep over for an unknown reason. One officer parked in front, while the second officer stopped behind the jeep, Lee said. The first officer approached the driver from the front on the driver's side door when the man, alone in his Jeep, fired a gun from his seat, Lee said. Moments later, the second officer approached from the rear passenger side and shot the man an unknown number of times, Lee said.

He returned to his car to radio in "officer down" as shocked motorists who witnessed the shooting phoned 911 for a fallen officer, Lee said.