"Investigators will work to identify the person or group responsible, the motivation and whether religious bias was a factor," the FBI said.
"All evidence will be reviewed by state and federal prosecutors to determine whether a violation under federal civil rights statutes occurred. The FBI is committed to ensure law-abiding citizens are protected and to deter those who would threaten them."
Hawthorne police said that after the San Bernardino attack, the department has communicating more closely with local Islamic centers.
"The Department will continue to closely support our faith-based partners and work to assure the safety of its members," Hawthorne police said.
CNN's Erica Henry contributed to this report.
'Jesus' spray-painted on two California mosques01:45
Vandals Spray-Paint ‘Jesus’ on 2 Mosques in California
A fake grenade was also left outside one of the mosques
(HAWTHORNE, Calif.) — The FBI is investigating after police in Southern California say two mosques in the city of Hawthorne were vandalized with paint and a fake grenade was left in the driveway of one of the properties.
The Hawthorne Police Department says it received a report early Sunday that a possible explosive device had been left outside the Ahmadiyya Muslim Community Baitus-Salaam Mosque. Officers found a replica of a hand-grenade and the word “Jesus” sprayed in white paint on the mosque’s metal gate.
The department says later Sunday it received a second call reporting the phrase “Jesus is the way” had been painted on the Islamic Center of Hawthorne’s facade.
Authorities on Friday arrested 23-year-old Carl James Dial Jr., of Palm Desert, in connection with a fire at the Islamic Center of Palm Springs.