AURORA, Colo. -- A plainclothes Aurora officer was gunned down and killed in broad daylight while he was sitting in his car, waiting at a stoplight Wednesday afternoon.

Officer Mike Thomas, a 24-year police veteran, was killed in the shooting that occurred at the Peoria and Montview intersection around noon.

Police arrested 27-year-old Brian Allen Washington, of Denver, at the scene, Aurora Police Chief Daniel Oates said at a news conference Wednesday night. Washington was being held at the Adams County Jail without bond on suspicion of first-degree murder.

Oates did not give a motive and did not say whether Washington knew Thomas was a police officer.

Oates said Washington has a significant criminal history and was being sought by Denver police for a shooting that occurred Monday at 33rd Avenue and Pontiac Street. In that Sept. 18 incident, a woman sitting inside a car was shot at. Police did not say if that woman was hurt.

Thomas was not in uniform at the time of the shooting but was technically on-duty when he was killed, police said. Thomas was in his own personal vehicle when a gunman walked up the driver's-side window and shot the officer at least once, 7NEWS reported.

Oates said Thomas was moving from one training site to another and was about to switch vehicles. Oates said Thomas was shot at least once, with a bullet entering his left side. He was pronounced dead at a nearby hospital.

Oates said a struggle took place prior to the arrest. He said there was no evidence that the incident involved road rage, as previously reported by some media outlets.

Oates said evidence will be presented to the Adams County District Attorney including a handgun that police recovered from the scene.

During the news conference, held in front of the fallen officers' memorial, dozens of police officers stood behind Oates. They had black tape across their badges and their faces were grim.

Thomas had been a canine officer and a narcotics officer before he was promoted to detective in April 2005 and worked in the 3rd District. He was a highly decorated officer who had received more than a dozen awards, including the Medal of Honor, from the Aurora Police Department, other law enforcement agencies and citizens.

"We are in mourning. This a terrible and tragic day for the department," said Oates. "He was beloved by the police department."

Thomas was described as being "among the hardest working detectives" and irreplacable.

"This is about as hard as it gets for us in policing," said Oates.

Flags were hung at half-staff.

Oates said the department was going to hold a private memorial with Aurora Police Officers after the news conference. olleagues said Thomas was well-liked, charismatic and was looking forward to retirement.

His desk was two away from where Officer Damon Vaz sits. "He's just a happy-go-lucky type of guy, very outgoing, opinionated, boisterous," Vaz said. "He's the type of guy people would gravitate to. Whenever you talked to Mike, he was just always, for the most part, in a good mood."

Oates also asked any witnesses who have not yet spoken to police to come forward and contact Officer R.J. Wilson at 303-739-6106. This shooting occurred in public in front of Fitzsimons Golf course and North Middle School. Witnesses to the Monday shooting at 33rd and Pontiac Street were also asked to call Denver police.

Washington was arrested with the help of citizens, another officer and two corrections officers.

Denver resident Paul Brandin, 51, said he drove up next to Thomas just after the incident occurred and saw that he was slumped over in his car, the Aurora Sentinel reported. Brandin said he thought the man had suffered a heart attack but then he noticed that several men wearing police weapon belts were struggling with a tall black male, trying to wrestle a gun from him.

Brandin got out of his car, and at one point during the struggle, the gunman dropped his weapon and Brandin kicked it under a car, he said. The men struggling with the suspect, who witnesses said were employees with the Federal Bureau of Prisons, then handcuffed the suspect, the newspaper reported.

After the shooting, about 50 police vehicles converged in the area, and Peoria was closed from East 23rd Avenue to East 17th Avenue for most of the day as detectives gathered evidence and interviewed witnesses.

It appeared that four cars were somehow involved in the shooting. Police officers deliberately obscured the cars' license plates with paper bags and cardboard.

A teal older-model Chrysler -- believed to be the officer's -- and a red two-door Toyota just to its left were in two northbound lanes on Peoria. A white van and a blue Cadillac were facing south on Peoria. The blue Cadillac and the blue older-model vehicle both had their windows shot out.

A pair of sneakers and socks, as well as some sort of other clothing and shattered glass, littered the street between the blue older car and the red car.

Police rounded up what appeared to be several potential witnesses and took them away from the scene, and others in the area said they were told by officers not to speak with reporters about what they saw.

Residents who live nearby said that they were scared but not surprised by the gunshots.

"It started about 1 or 2 o'clock. I wasn't really paying attention. Like I said, we hear them quite often so it's nothing new to hear anything. But then all of a sudden we heard fire trucks, we heard the ambulance," said Rhonda, who didn't want her last name released.

The scene is across the street from North Middle School, which was not placed on lockdown because a suspect was already in custody, school officials said. When students were released at 2 p.m., they were escorted single-file away from the crime scene, and there were no disruptions to the classes, 7NEWS reported.

Washington will appear in court Thursday at 1:30 p.m. Washington has previous charges for marijuana and narcotics possession.

Police spokesman Bob Friel said Thomas' death was the first in the line-of-duty for the department since the 1980s.