Police are searching for a white male after a homemade bomb exploded outside the building of a black civil rights group in Denver.
No one was injured in the blast outside the Colorado Springs chapter of the National Association for the Advancement of Colored People (NAACP) and the building only sustained minimal damage.
An "improvised explosive device" was set off against an exterior wall of the building, but a gasoline can placed next to the bomb failed to ignite, according to the FBI's Denver field office.
Authorities are now searching for a man described as balding, Caucasian and about 40 years old.
An FBI statement added: "He may be driving a 2000 or older model dirty, white pick-up truck with panelling, a dark coloured bed liner, open tailgate and a missing or covered license plate."
The FBI have said investigations are under way to determine whether the civil rights group were deliberately targeted or whether race had anything to do with the incident.
Chapter president Henry Allen Jr told The Colorado Springs Gazette said he was hesitant to call the incident a hate crime, but added the incident would not "deter us from doing the job we want to do in the community".
The NAACP is the oldest civil rights organisation in the US after being founded in 1909.