A Colorado State Patrol trooper was on the shoulder of Interstate 25 near Castle Rock when a tractor-trailer rig crossed the solid white line and hit him, killing him instantly, according to court documents obtained by 9NEWS.

Trooper Cody Donahue, an 11-year veteran, was out of his patrol car investigating another crash when he was killed shortly before 2 p.m. Friday. He had returned to a job working on the streets just three weeks before.

Noe Gamez-Ruiz, 41, the driver of the truck, was arrested on suspicion of careless driving causing death and failure to yield the right of way to an emergency vehicle.

"These aren’t even misdemeanors, they’re misdemeanor traffic offenses, which is even lower than a misdemeanor," George Brauchler, District Attorney for the 18th Judicial District said. "Each of these charges carries with it the max potential sentence of a $1,000 fine and one year in the county jail. But it’s unlikely those would ever be stacked, as it grows out of the same episode of criminal conduct and frankly that’s what the law provides."

Gamez-Ruiz had been a driver for US Foods for more than 15 years. All US Foods drivers are vetted through a robust application process and take part in ongoing safety training, according to a statement from US Foods.

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According to an arrest affidavit, Donahue and a fellow trooper, Matthew Normandin, were called to investigate a crash on northbound I-25 at the south end of Castle Rock. Both troopers were in marked patrol cars, parked on the right shoulder, and had their rear-facing emergency lights operating, according to the affidavit.

Normandin was in his patrol car when Donahue walked up to the wrecked vehicle to complete paperwork to impound it. Traffic at the time was traveling between 55 and 65 mph, Normandin estimated, according to the affidavit.

Normandin told investigators he looked up and saw Donahue standing next to the wrecked vehicle. At that point, he saw a truck “cross the solid white fog line” – the stripe along the right side of the road separating the shoulder from the outside lane – and hit Donahue, killing him instantly.

According to the affidavit, video from Donahue’s patrol car and the truck showed that it “drifted over the fog line” and hit the officer. According to the affidavit, Gamez-Ruiz had room to move to the other lane but did not.

Gamez-Ruiz agreed to have his blood drawn to check for the presence of drugs or alcohol – a typical measure in a serious crash.

"One thing the public can do that will make a difference is when you see cars on the shoulder of the road, when you see emergency lights activated, get out of the way," Brauchler said. "Put yourself in a position to do no harm as you drive by these people, we need more of that."

Donahue went to work on the streets three weeks before his death. Before that, he work at the Colorado State Capitol, but wanted to get back on the road.

According to an obituary posted online Monday, Donahue’s funeral service will be held at 11 a.m. Friday at the Denver First Church of Nazarene at 3800 E. Hampden Ave. in Englewood.

In lieu of flowers, memorial contributions can be made in Donahue’s memory to the Flying Wheels Foundation at 15055 S. Golden Rd. in Golden.

A GoFundMe page has been set up for Donahue’s family: http://bit.ly/2fXCUqo.

Contact 9NEWS reporter Kevin Vaughan with tips about this or any story: kevin.vaughan@9news.com or 303-871-1862.