Nothing in the Quran, Islam's holy book, strictly bars portrayals of Mohammed. But the faith, like the Hebrew Bible's Ten Commandments, has long discouraged any graven images, scholars say, to avoid the temptation toward idol worship.
In some ways, early Muslims were reacting to Christianity, which they believed had been led astray by conceiving of Christ not as a man but as a God. They did not want the same thing to happen to Mohammed.
"The prophet himself was aware that if people saw his face portrayed by people, they would soon start worshiping him," Akbar Ahmed, who chairs the Islamic Studies department at American University, told CNN. "So he himself spoke against such images, saying 'I'm just a man.' "
In a bitter irony, the recent violent attacks against portrayals of the prophet are kind of reverse idol-worship, revering -- and killing for -- the absence of an image, said Hussein Rashid, a professor of Islamic studies at Hofstra University in New York.
While the Quran does not explicitly prohibit depictions of Mohammad, most contemporary Muslims worldwide abide by the ban, based largely on religious rulings by Islamic scholars.
Sheikh Yasir Qadhi, a Muslim scholar who lives in Tennessee, said Islamic law draws a distinction between making graven images, a statute that is not binding on non-Muslims, and portraying Mohammed in a vulgar or disrespectful manner, which is considered blasphemy.
In the West, free-speech advocates such as the French magazine Charlie Hebdo have often done both, depicting Mohammed as foolish or bent on violence.
"In the context of Europe, where in many countries Muslims feel like they are besieged, these images are not seen as criticism but as bullying," said Rashid. "Violence as a response is clearly wrong and disproportionate. However, it is not so much about religious anger as it is about vengeance."
But even in the United States, where Muslims are relatively acclimated, extremists have opposed the portrayal of Mohammed on "South Park," the satirical cartoon show, and the subsequent "Draw Mohammed Day,"
that erupted in response.
Ban includes Jesus and Moses
Mohamed Magid, an imam and former head of Islamic Society of North America
, told CNN that the Muslim prohibition on depicting prophets extends to Jesus and Moses, whom Islam treats as prophets. Some Muslim countries banned the films "Noah"
this year because their leading characters were Hebrew prophets.
In Sunni mosques, the largest branch of the faith, there are no human images of any kind. The spaces are instead decorated with verses from the Quran.
But there have been historical instances of Muslims depicting the prophet, especially in Shiite branches of Islam, Omid Safi, a religious studies professor at Duke University, told CNN.
"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," said Safi. "The one significant context where depictions of the prophet have not been image-related has been in the Arab context."
Johari Abdul-Malik, the imam for Dar Al-Hijrah Islamic Center in Falls Church, Virginia, told CNN that depictions of the prophet 's teachings were sometimes used to bridge gaps in illiteracy.
Even historical renditions of Mohammed by Muslim artists were careful not to paint the prophet in too much detail.
Mohammed shown only in shadow
For example, Ahmed told CNN 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."
In a Muslim film called "The Messenger," which circulated throughout the Muslim world in the 1970s and 1980s, Mohammed was shown only in shadow.
In the Quran, there is "no statement from the prophet requesting his image not be recorded," Abdul-Malik told CNN.
Instead, the teaching about images comes from the hadith
, a record of the sayings and actions of the Prophet Mohammed and his closest companions. The hadith is considered secondary only to the Quran in terms of textual authority, but the sometimes contradictory accounts have led to centuries of debates within the umma, or global Muslim community.
Scholars of religion say opposition to portraying Mohammed wasn't generally violated in earlier centuries because of a gulf between Western and majority-Muslim nations.
In the age of globalization, non-Muslims and critics of Islam have felt free to depict Mohammed, including in offensive ways. In 2006, for example, a Danish cartoonist's depiction of the prophet wearing a bomb as a turban with a lit fuse sparked demonstrations across the world.