Denver District Attorney Mitch Morrissey will not pursue criminal charges against a Thornton police detective who shot an Adams County sheriff’s deputy in December while trying to apprehend a murder suspect.

There are no applicable criminal charges to file against Thornton Police Det. Brent Mullen, who fired the shots, Morrissey wrote in his decision letter released Wednesday. The decision does not limit the Thornton Police Department’s ability to discipline Mullen.

The decision letter gives a lengthy description of the search for a murder suspect, who was considered dangerous, that led undercover officers from Thornton to Denver. Mullen shot Adams County Sheriff Department Det. Mike Robbins, who was chasing the suspect on foot through an alley.

The shooting happened Dec. 12 near the intersection of West 40th Avenue and Bryant Street in Denver. Officers from multiple agencies had descended on a house to serve an arrest warrant on Furmen Lee Leyba, who was wanted in connection with the shooting death of three people Adams County.

Undercover officers had followed Leyba to Denver by tracking his cell phone. They followed him as he rode on a motorcycle with a woman through the streets of northwest Denver, and they eventually surrounded a house where the two had stopped.

When Leyba and the woman left that house on their motorcycle, Robbins followed them. He pulled in front of the motorcycle at the intersection of West 39th Avenue and Bryant Street.

The two hopped off the motorcycle and ran in different directions with Robbins chasing Leyba, who dropped a handgun after he tripped in a driveway.

The chase led to a dead-end alley where Mullen was parked in an unmarked pickup truck with a Thornton police sergeant .

The two drew their guns when they saw Leyba emerge from behind a fence and run toward their truck. Leyba surrendered.

But Robbins was close behind and had his gun drawn, the letter said.

Mullen told internal investigators that he saw the other person , who was wearing dark clothes and carrying a silver gun, and believed they were getting ambushed. Moments before the shooting, he and the sergeant had discussed the possibility of an ambush and their vulnerable position in the alley.

Mullen was quoted in the letter as saying, “I remember thinking, ‘He’s going to start shooting me through the windshield! He’s going to start shooting me through the windshield!’ And I, I start shooting through the windshield.”

But the sergeant with Mullen recognized the Adams County deputy as a fellow officer and began shouting that he was “friendly,” the letter said.

Robbins was not wearing clothes or badges that identified him as a police officer, the decision letter said

One bullet struck Robbins in the lower left arm, breaking a bone. He was treated and released at a hospital the night of the shooting.

Leyba, 38, was captured at the scene. Gabriel Flores also has been charged in the triple homicide. Both remain jailed in Adams County.

In concluding the letter, Morrissey wrote that Mullen had reasonable belief that he and his partner were at risk of being shot.

“It is worth noting that each of the officers involved in this incident were courageous in their attempts to apprehend a violent and dangerous criminal,” the letter said. “We are gratified they were successful in that effort and grateful the injuries Det. Robbins sustained were not life threatening.”

Mullen had been assigned to a desk job, pending the investigation. He returned to his duties as a detective on Wednesday after he was cleared of criminal charges, said Officer Matt Barnes, a Thornton police spokesman.

An administrative review is ongoing and should be completed soon, Barnes said. The department will review its undercover training and equipment in an effort to improve safety.

Noelle Phillips: 303-954-1661, nphillips@denverpost.com or twitter.com/Noelle_Phillips