"The Sikh Gurdwara in Riverside was vandalised on the night of July 29 in an apparent hate crime. The word 'terrorist' was spray-painted around the complex," the Sikh American Legal Defence and Education Fund (SALDEF) said.
The group said it has been in direct contact with Riverside County Sheriff Department, the Riverside Police Department, the FBI and Department of Justice CRS urging them to investigate the incident as a hate crime.
A year ago on August 5, 2012, six Sikh worshippers were shot dead by a US Army veteran and white supremacist, Wade Michael Page, who went on a shooting rampage in the Gurdwara in Oak Creek.
"Attacks and vandalism against any of the nation's houses of worship must be condemned by all Americans. These acts strike the very foundation of religious tolerance, a fundamental freedom this country was built upon," he said.