Trooper Sean O'Connell was directing traffic on his motorcycle about 5:45 p.m. assisting with a detour around the Skagit River Bridge collapse.
That's when he was hit by a box truck near Fir Island Road and Main Street in Conway, according to WSP Chief John Batiste.
Details of the fatal collision are under investigation, but it is known that O'Connell had checked the length of a vehicle backup and was returning to a traffic control point when the fatal collision occurred, the State Patrol said.
Witness Tom Wuellner was driving through Conway just after the collision and said he saw a tipped over motorcycle and an injured trooper on the ground.
Wuellner said several people were already tending to the injured trooper. After passing the crash scene, Wuellner said he spotted another trooper up the road.
"I ran up to his partner and said, 'Your partner is down, your partner is down. You need to get back there,' and he ran back there," Wuellner said.
O'Connell, 38, was taken to a hospital, where he succumbed to his injuries.
"Needless to say, this is a very difficult time," Batiste said. "Did I say we lost a great trooper? We lost a great human being."
Troopers say the truck driver, a 49-year old from Mount Vernon, is cooperating with the investigation and had no signs of impairment.
"We have no indication that this is anything other than a tragic accident," Batiste said.
Batiste said O'Connell, who was a 16-year veteran of the state patrol, had previously been named Trooper of the Year for his district.
"He was a very hard working trooper who did his job well in service to the people of this great state," he said.
O'Connell is only the second trooper to be killed in the line of duty since 1999, and the first since trooper Tony Radulescu was gunned down in February 2012.
A Navy veteran, O'Connell is survived by his wife and two children, ages 5- and 7-years old, according to Batiste said.