When a Jefferson County deputy unleashed pepper spray at unruly protesters on the first night of the Democratic National Convention, he did not know that his targets were undercover Denver police officers.

Now the American Civil Liberties Union of Colorado is questioning whether that staged confrontation by police pretending to be violent inflamed other protesters or officers during the most intense night of the four-day event.

The protest occurred Aug. 25 at 15th Street and Court Place near Civic Center. Police ultimately arrested 106 people, the highest number of arrests in a single day during the convention.

According to a use-of-force police report obtained by the ACLU, undercover Denver detectives staged a struggle with a police commander to get pulled out of the crowd without blowing their cover. The commander knew they were working undercover, and the plan was to pull them out of the crowd and pretend they were under arrest so protesters would be none the wiser.

A Jefferson County deputy, unaware of the presence of undercover police, thought that the commander was being attacked and used pepper spray on the undercover officers.

The report says that the commander and an undercover detective were sprayed, but it does not indicate how many others were affected. The report also doesn't say whether the pepper spray used on the undercover police was the first deployment of chemicals that night or whether the riot was already underway.

Denver police have said they were trying to control the crowd moving from Civic Center. The officers testified in court that they had intelligence that anarchists planned to gather in the park, then move toward the 16th Street Mall to wreak havoc at delegate hotels and other businesses. The activists had posted that plan on a publicly available website.

Probe requested

On Thursday, the ACLU of Colorado sent a letter to Denver's Independent Monitor, Richard Rosenthal, asking for the Internal Affairs Bureau to conduct an investigation of the pepper-spraying incident.

"The actions of the undercover detectives on August 25, 2008, may have had the effect of exacerbating an already 'tense situation,' as their feigned struggle led nearby officers and the public to believe that a commanding officer was being attacked by protestors and that the situation necessitated the use of chemical agents," says the letter, written by ACLU staff attorney Taylor Pendergrass.

"Such actions may have escalated the overall situation by causing officers on the scene to fear that the protestors threatened their safety, when in fact the struggle was only between uniformed officers and undercover officers," he wrote.

Denver Police Chief Gerald Whitman did not return a call seeking comment about the pepper-spray incident and whether the officers followed protocol by staging a disturbance with the commander.

Rosenthal said he had received the ACLU's letter about the pepper-spray incident.

He also received a letter from the ACLU last week requesting a probe into possible conflicting or false statements by police about the riot and whether the department withheld evidence in some of the protesters' criminal trials.

The ACLU contends videos show that protesters, as well as otherwise uninvolved onlookers, were never ordered or given a chance to disperse before they were surrounded and detained by police.

"The letters have been received, and I am in the process of reviewing and evaluating them," Rosenthal said Thursday.

As many as 60 protest suspects declined to accept plea deals after their arrests. Some cases have been dismissed and some suspects acquitted after a judge cited a lack of evidence.