Police and the FBI confirmed they were reviewing the incident, though they said it was too early to discuss potential charges.
"We've got to be involved," said Officer Pete Berndlmaier of the 26th District, who gathered information at the scene. "If they get away doing something like that, they are going to up the ante."
Police retrieved the surveillance video and interviewed potential witnesses. In the video, which was viewed by The Inquirer, the truck's license plate is not easily discernible, but officers said authorities might be able to enhance the video.
Rue Landau, executive director of the Philadelphia Commission on Human Relations, said the incident could be a violation of the city's ordinance on ethnic intimidation and institutional vandalism.
"It is a heinous act that sends a message to Arab American communities that they are not wanted here in Philadelphia, and that could not be further from the truth," Landau said.
Mayor-elect Jim Kenney expressed outrage.
"The bigotry that desecrated Al Aqsa mosque today has no place in Philadelphia," he said in a statement. Philadelphia "has a long history of coming together in the face of challenge. We cannot allow hate to divide us now, in the face of unprecedented difficulties. I ask all Philadelphians to join me in rejecting this despicable act and supporting our Muslim neighbors."
Marwan Kreidie, who as head of the Arab American Development Corp. maintains an office at the mosque, reported the incident to the FBI in Philadelphia. FBI spokeswoman Carrie Adamowski said agents were investigating.
Kreidie said the terror attacks in Paris and San Bernardino, Calif., along with the anti-immigrant rhetoric of some politicians, have ramped up hostility against Muslims.
"It's worse now that it was after 9/11," he said, "which is really frightening. [Sept. 11] was a horrible incident, but we didn't have this kind of reaction, nobody threw a pig's head. . . . A pig's head doesn't do much. Could the next thing be a pipe bomb?"
Nabil Ibrahim Khalil, the caretaker at the mosque since it opened 23 years ago, said an anonymous caller left a voice mail message the day after the Paris terrorist attacks last month in which he called Allah "a piece of pork s--." The answering machine trapped the male caller's number, which was turned over to police.
Khalil said he was unsure if the incidents were related.
Lt. John Stanford, spokesman for Philadelphia police, said officers would try to determine if charges are warranted. "Obviously, people shouldn't be leaving pigs at houses of worship," he said.
Landau said a balancing act is necessary when deciding how much to publicize such incidents.
People should be informed of the incidents, she said, but without multiplying "the negative effect by giving extra press to the . . . hateful acts."
Inquirer staff writer Aubrey Whelan contributed to this article.