Both acts of vandalism took place in the same California city, about an hour west of where the San Bernardino terror attack
took place this month.
And Hawthorne police said both incidents are now classified as hate crimes.
Early Sunday morning, Hawthorne police received a call about vandalism and a possible explosive device in front of the Ahmadiyya Muslim Community Baitus-Salaam Mosque, the Police Department said.
When officers arrived, they found what looked like a grenade and evacuated the area. But the object turned out to be a fake plastic grenade.
Someone had also spray-painted "Jesus" across the mosque's fence.
The same day, another mosque -- the Islamic Center of Hawthorne -- was found vandalized, with "Jesus is the Way" spray-painted on the front of the building.
Hawthorne police believe both crimes took place overnight.
The FBI and the Los Angeles County Sheriff's Department are working with Hawthorne police in the investigation.
"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.