A severed pig's head was left outside Philadelphia mosque. An Islamic center in Florida was defaced. A Sikh temple in California was vandalized by someone who mistook it for a mosque.

Advocacy groups believe there has been a spike in anti-Muslim incidents across the United States in recent weeks.

They've linked it to last week's mass shooting in California and the inflammatory rhetoric of Donald Trump and other Republican presidential candidates.

And they say that Muslims are fearful the backlash could lead to further harassment and violence.

The Anti-Defamation League says it has tracked more than three dozen anti-Muslim incidents since the Nov. 13 terror attacks in Paris that left 130 dead.

Advocates said Wednesday they believe the pace has picked up since the San Bernardino shootings that killed 14.