Vandals sprayed "Hale Satan" and other profanities along with what appeared to be a penis on the Grand Mosque overnight and the imam, Hassan Ahmed, found it early Saturday, The Oklahoman of Oklahoma City said.
"We are certainly aware of that incident [Saturday] morning, and we have been notified, and are investigating to determine if it does fit the criteria for a hate crime," FBI spokesman Rick Raines said. "We're investigating it as a potential hate crime."
It was unclear if the incident was in retaliation for the bombing of the Boston Marathon. Two brothers of Chechnyan descent who are Muslims are suspected of carrying out the Boston attack.
The mosque enhanced security by installing surveillance cameras in 2011 after vandals sprayed it with paintballs. Hassan said the vandals this time avoided detection by cameras, leading him to believe they had studied the building beforehand.