Detectives still aren't sure whether the trooper was trying to pass the truck on his motorcycle before it made a right turn and the collision occurred.
O'Connell, 38, died May 31 in an on-duty collision while working traffic control related to the Skagit River bridge collapse. The Lake Stevens-area man was a 16-year veteran of the State Patrol.
Detectives from the State Patrol determined that a combination of factors led to O'Connell's death.
"There was no one single cause for this tragedy," said Capt. Charles LeBlanc, commander of the Patrol's Criminal Investigation Division. "There was a combination of circumstances that led to a horrific end."
The collision investigation is continuing, but detectives have determined the fundamental facts of what occurred, LeBlanc said.
Shortly before the collision, O'Connell checked the length of a traffic backup on the Skagit River Bridge detour and was returning to where another trooper was directing traffic. He was moving up the shoulder, outside the fog line, on the right of the box truck.
As the two vehicles approached the intersection of Fir Island Road and Greenfield Street south of Mount Vernon, the box truck made a right turn onto Greenfield and the truck was struck on the right side by O'Connell's motorcycle. The box truck driver did not see O'Connell's motorcycle before he began his turn.
Speed was not a factor in the collision. Both vehicles appear to have been traveling below the 35 mph speed limit, detectives determined.
The truck driver was not cited and the patrol said it is not pursuing charges against anyone. The truck driver agreed to a voluntary blood test for alcohol or drugs, and there is no evidence he was impaired in any way. He cooperated fully in the investigation.
Another factor in the fatal crash was a guard rail to the trooper's right that cut off O'Connell's avenue of potential escape.
After striking the box truck, O'Connell fell underneath it.
"A couple of seconds in time, or a couple of feet either way, and we'd likely have had a very different outcome," LeBlanc said.
"Unfortunately, it was the wrong location at the wrong time," he added.
LeBlanc said that O'Connell was not using his emergency lights and siren at the time of the collision, in keeping with agency training. Experience has shown that some drivers, upon hearing a siren, make sudden maneuvers to the right. If troopers are forced to pass on the right, they generally turn off their emergency lights and sirens to avoid being hit.
Troopers are taught how to pass other vehicles using the shoulder and to consider escape routes.
"The reason we have motorcycles is precisely because they can maneuver around traffic," LeBlanc said. "Passing on the right is something our troopers are trained to do in the performance of their duties, and like many of our duties, it involves risks."
The bridge collapsed May 23 after being struck by a truck with an oversized load. The broken bridge created massive backups along the busy I-5 corridor.
The State Patrol's Major Accident Investigation Team, made up of its most experienced collision detectives, spent months trying to piece together what caused the fatal collision involving the motorcycle and box truck, which was carrying paper fliers.
The investigation so far has taken nearly six months because there was no evidence of an obvious cause, officials said.
Typically, accident investigators examine human, engineering and mechanical factors.
In many accidents, there are signs of alcohol or drug-impaired driving, speeding or an unfastened seatbelt that make it easier to determine what went wrong. That wasn't the case in O'Connell's death, leaving detectives to try to sort through other possible contributing factors.
LeBlanc said there is no timeline for completing the investigation.
O'Connell was the 28th trooper to die in the line of duty. He also is one of more than a dozen law enforcement officers from Snohomish County to die on duty in the state since 1902. More than 2,200 people, including many troopers and police officers from out of state, attended his funeral at Comcast Arena in Everett.
Local and state government leaders have renamed landmarks in O'Connell's honor.
The Washington State Transportation Commission approved a measure to rename the Skagit Bridge the "Trooper Sean M. O'Connell, Jr. Memorial Bridge."
The city of Lake Stevens, meanwhile, is naming a street after the fallen trooper.
It chose 83rd Avenue SE. The street also will keep its original name to avoid confusion, especially for emergency responders and delivery services.
It has been a tough investigation on many levels, LeBlanc said. Many of the detectives knew O'Connell.
"Most of the time, it's easy to detach ourselves if we don't have an emotional tie," LeBlanc said. "This case was like a family member. It was extremely, extremely difficult."
Eric Stevick: 425-339-3446, email@example.com
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