Thornton Mother’s 2003 Murder: Break in Cold Case After 22 Years

A decades-old Colorado cold case might just one step closer to being solved.

How Many Cold Cases Remain Unsolved in the U.S.?

Between the years of 1965 to 2023, approximately  345,000 cases of homicide and "non-negligent manslaughter" went unsolved, according to the official Project Cold Case website.

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Who Was Rosa Arguello?

In 2003, a 29-year-old from Colorado named Rosa Arguello was found dead from multiple stab wounds in Thornton, according to CBS reports.

The Night of the Murder in Thornton

Her remains were found on the side of the road in the 12000 block of Quebec Street on October 17th of that year.

Arguello was living in Thornton with her children at the time. She also was a member of the Hopi Tribe.

Read More: Body Found in Horsetooth Reservoir Search

Break in the Thornton Murder Case: Suspect Arrested After 22 Years In Iowa

For nearly 22 years, her case ran cold. However, a possible suspect in this murder case was arrested hundreds of miles away in the Midwest.

On Monday, September 15th 2025, law enforcement from Dubuque, Iowa arrested 55-year-old Robert McClain. This was on a homicide warrant out of Adams County. Officials searched his home and found him in possession of a firearm as a felon, according to reports.

Who Is Robert McClain?

This man has a criminal history of disturbing acts of violence, reports say.

In February of 2014, he was charged with attempted murder after attacking his ex-wife. McClain pleaded guilty to some lesser charges and was sentenced to 15 years in prison.

Thornton Police Respond to Arrest in Iowa

"We are grateful for our partners at Dubuque PD and will continue to work closely with the 17th Judicial District Attorney's Office to pursue justice for Rosa and her family," said Thornton Police Chief Jim Baird.

Read More: Arson at a Loveland Charlie Kirk Memorial Leads to Arrest

Multi-State Investigation Into Colorado Cold Case

Apparently Thornton law enforcement have been working with Dubuque Police to investigate this suspect since October of 2024, according to a report from Iowa News Now.

The Thornton police worked with this Iowa police force to apprehend McClain.

He is currently being held at the Dubuque Police Department in Iowa.

This is still an open investigation, we will keep you updated as more information comes out.

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