Violence over depictions of the Prophet Mohammed may mystify many non-Muslims, but it speaks to a central tenet of Islam: that the Prophet was a man, not God, and that portraying him threatens to lead to worshiping a human instead of Allah.
"It's all rooted in the notion of idol worship," says Akbar Ahmed, who chairs the Islamic Studies department at American University. "In Islam, the notion of God verses any depiction of God or any sacred figure is very strong."
"The Prophet himself was aware that if people saw his face portrayed by people, they would soon start worshiping him," Ahmed says. "So he himself spoke against such images, saying 'I'm just a man.'"
The prohibition against such portrayals was on stark display Tuesday, as mobs in Egypt and Libya attacked U.S. compounds in response to a film that vilifies the Prophet Mohammed, who founded Islam in the 7th century.
The attack on the U.S. Consulate in the Libyan city of Benghazi killed J. Christopher Stevens, Washington's ambassador to Libya, as well as three other Americans at the compound.
The film that's believed to have inspired the violence depicts the Prophet Mohammed as a child molester, womanizer and ruthless killer, going a big step beyond violating the basic Muslim prohibition against depicting the Prophet, even in a favorable light.
There are questions about who is behind the movie. Initial reports identified a supposedly Israeli-American real-estate developer named Sam Bacile, but it's unclear if that person even exists. A member of the film's production staff told CNN that the producer's name was listed as Abenob Nakoula Basseley.
In mosques, there are no images of people of any kind. The spaces are often decorated with verses from the Quran.
Mohamed Magid, an imam who leads the Islamic Society of North America, says the Muslim prohibition on depicting prophets extends to Jesus and Moses, who Islam treats as prophets.
"Pictures and images are prohibited from being worshiped," Magid says.
There have been historical instances of Muslims depicting the Prophet, says Omid Safi, a religious studies professor at the University of North Carolina who has studied the issue.
"We have had visual depictions of the Prophet in the form of miniatures and pictures in the Iranian context, the Turkish context, the central Asian Context," says Safi, author of the book "Memories of Mohammed." "The one significant context where depictions of the Prophet have not been image-related has been in the Arab context."
But even depictions of the Prophet by Muslim artists has been a sensitive issue.
Akbar, a former Pakistani ambassador to the United Kingdom, says that Muslim artists in the 15th and 16th centuries would depict the Prophet but took pains to avoid drawing his face.
"It would be as if he was wearing a veil on his face, so the really orthodox could not object -- that was the solution they found," Akbar says.
In a Muslim film called "The Messenger," which circulated throughout the Muslim world in the 1970s and 1980s, the Prophet is depicted only as a shadow.
Scholars of religion say Muslim opposition to portraying Mohammed wasn't generally violated in earlier centuries because of a gulf between much of the Muslim world and the West.
In the age of globalization, non-Muslims and critics of Islam have felt free to depict Mohammed, including in offensive ways.
In 2006, a Danish cartoonist's depiction of the Prophet wearing a bomb as a turban with a lit fuse provoked demonstrations across the world.
Akbar says that until relatively recently, depictions of Jesus tended to be reverential, but Christianity has had a decades-long head start in dealing with negative portrayals of Jesus in film and art.