The Red Shirt movement is loyal to Thai Prime Minister Thaksin Shinawatra, who was ousted in a 2006 military coup and lives in self-imposed exile, mostly in Dubai. Clashes between Red Shirt supporters and security forces left at least 89 people dead in 2010.
A 275-page report from a truth and reconciliation commission said both sides were likely responsible for the escalation of violence in 2010. The BBC reports, however, that the commission lacked the authority to subpoena witnesses and therefore wasn't able to indicate which side was responsible for the deaths.
Robert Amsterdam, legal counsel for the Red Shirt movement, said the report was an insult to those caught up in the 2010 violence.
"The Red Shirts reject the conclusions of this biased report as another attempt to whitewash the crimes against humanity committed by a small group of Thai elites," he said in a statement from Washington.
A breakaway general was assassinated during a live media interview amid the unrest in 2010.
The BBC reports that the human rights community blames the use of live ammunition by government forces for much of the deaths. Members of the protest movement, however, were likely responsible for some deaths as well.