"What do you mean robbery?" FourHorn remembers asking the officers. Her only brushes with the law had been a few speeding tickets.

She was locked up in a Colorado jail. They took her clothes and other belongings and handed her an oversize black-and-white striped uniform. She protested for five days, telling jailers the arrest was a mistake. Finally, her husband borrowed enough money to bail her out.

"They wouldn't tell me the details," she said.

Later, it became clear that FourHorn was right, that Denver police had arrested the wrong woman. Police were searching for Christin Fourhorn, who lived in Oklahoma.

Their names were similar, and Christina FourHorn, a mother with no criminal record living in Sterling, Colorado, had been caught in the mix-up.

FourHorn went public about her case more than two years ago, filing a lawsuit that alleged the arrest violated her constitutional rights. The Fourth Amendment protects citizens from arrest without probable cause.

The problem of mistaken arrests continues, said attorneys with the American Civil Liberties Union of Colorado. The group, which represented FourHorn, calls Denver's police work "recklessly sloppy." An ACLU mistaken identity lawsuit on behalf of four other people is pending against Colorado police agencies.

A mistaken identity arrest occurs almost every day, said policing experts and officials at the National Association of Criminal Defense Lawyers. But most people taken into custody are released shortly after the mistake is realized.

Since the FourHorn case, the ACLU found at least 237 cases in Colorado in which police may have arrested the wrong person. The figure is likely a small sample since police often release those wrongfully arrested before the first court appearance, the ACLU said.

"We are trying to demonstrate that this is a widespread practice," said Mark Silverstein, an ACLU attorney who filed FourHorn's suit in 2008. FourHorn's case was settled, and the terms remain confidential.

"This is not some fluke in a rational system," Silverstein said. "It's something that happens regularly, predictably, and therefore the city should be doing more to ameliorate the problem."

Silverstein said his search of Colorado court records showed repeated examples of police arresting the wrong person:

"Defendant states this is not him and he has never driven a car!!!!" said one.

"Dismissed, wrong defendant. Sister used her ID," another said.

In 2009, Denver's Department of Safety found 51 cases in which a person claimed the warrant naming them was incorrect -- a number that's a small fraction of the 46,864 people arrested that year. A Denver police spokesman declined to comment on the mistaken identity arrests.

"While no one should be misidentified and incorrectly held in jail, we realize it can happen," said Mary Dulacki, records coordinator for Denver's manager of safety.

Experts at the Legal & Liability Risk Management Institute said name similarities such as in the FourHorn case are a common reason for errors. The group, based in Indianapolis, Indiana, trains police departments across the country on how to avoid mistaken arrests.

Other times, police may be relying on a person's alias. Suspects often give officers false names, which remain on their records as an alias. Also computer typos and glitches lead to mistaken identity arrests, policing experts said.

An alias mistake allegedly occurred in March 2007 when Denver police arrested Muse Jama, a college student studying for an exam, under a warrant for a person named Ahmed Alia. Jama's name had popped up as one of Alia's aliases.

Jama protested and showed the officers his identification cards. Still, he was arrested and remained behind bars for eight days. His lawsuit against the Denver Police Department, filed in 2008, is pending.

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