NEW YORK -- A severed pig's head was left outside a mosque in Philadelphia. An Islamic center in Florida was defaced. A Sikh temple in California was vandalized by someone who mistook it for a mosque and left graffiti that included a profane reference to the Islamic State group.

Advocacy groups said a spate of anti-Muslim incidents across the United States in recent weeks can be linked to last week's mass shooting in California and the inflammatory rhetoric of Donald Trump and other Republican presidential candidates. And they said Muslims are fearful the backlash could lead to further harassment and violence.

"The spike began with the Paris attacks and has intensified with what happened in San Bernardino and now with what Donald Trump is proposing," Ibrahim Hooper, lead spokesman for the Council on American-Islamic Relations, said Wednesday. "I have never seen such fear and apprehension in the Muslim community, even after 9/11."

It's hard to measure the extent of the problem -- or to know for sure whether there has been an increase in incidents or people simply are paying closer attention to a small but steady stream of episodes that occur throughout the year.

The FBI, which keeps statistics on hate crimes committed nationwide, counted 154 bias offenses against Muslims last year. Data for 2015 is unavailable.

The Anti-Defamation League, relying partly on complaints and partly on media reports, said it has logged more than three dozen incidents since the Nov. 13 terror attacks in Paris that left 130 dead.

"We're talking at least three dozen that we're aware of, and I'm sure there are many more incidents that haven't been reported," said Oren Segal, the director of the ADL's Center on Extremism.

"With legit terror attacks and the public discourse about them, it has created an atmosphere ripe for these types of stereotypes and incidents," Segal said. "People are exploiting them."

Segal wasn't able to provide a comparison point to the same period last year but said the pace of incidents appears to have picked up since the Dec. 2 shooting in San Bernardino that killed 14 people and injured 21 others. The suspects, Syed Rizwan Farook and his wife, Tashfeen Malik, later died in a gun battle with police.

That shooting prompted Trump on Monday to propose a complete ban on Muslim immigrants into the U.S., triggering a fierce debate that has dominated the national political conversation.

Advocates said other GOP presidential candidates also have fueled anti-Islamic sentiment, including Ben Carson, who suggested a Muslim should not be president, and Rick Santorum, who questioned whether the U.S. Constitution protected Islam.

Among the incidents reported since the California shooting:

• In Philadelphia, police have stepped up patrols around houses of worship after a severed pig's head was left outside the Al Aqsa Islamic Society.
• A Florida man was arrested after authorities said he vandalized the Islamic Center of Palm Beach in North Palm Beach, Florida. More than a dozen windows were broken.
• The imam of an Islamic center in St. Louis told a local television station someone claiming to be a former Marine left a threatening voicemail message on the mosque's answering machine Saturday. KTVI, which obtained a copy of the recording, reported the man said he "killed a lot of Muslims" and, using expletives, said he would decapitate Muslims.
• The Washington headquarters of the Council on American-Islamic Relations was evacuated Thursday afternoon after staffers came in contact with a suspicious substance that was mailed to the office. Hooper said the employee opened an envelope with a white, powdery substance Thursday afternoon. The group's executive director said one employee was coughing and another complained of a headache after the envelope was opened. It was not immediately clear whether the symptoms were related to the substance.