BENGHAZI, Libya – The U.S. dispatched an elite group of Marines to Tripoli on Wednesday after the mob attack that killed the U.S. ambassador and three other Americans. Officials were investigating whether the rampage was a backlash to an anti-Islamic video with ties to Coptic Christians or a plot to coincide with the anniversary of 9/11.

Tuesday’s stunning attack on the American Consulate in Benghazi poses a daunting task for U.S. and Libyan investigators: Searching for the culprits in a city rife with heavy weapons, multiple militias, armed Islamist groups and little police control.

The one-story villa that serves as the consulate was a burned-out wreck after the crowd armed with machine guns and rocket-propelled grenades rampaged through it. Slogans of “God is great” and “Muhammad is God’s Prophet” were scrawled across its scorched walls. Libyan civilians strolled freely in charred rooms with furniture and papers strewn everywhere.