A guide to the controversy:
SETTING THE SCENE
The 2011 revolt that deposed and killed Libyan dictator Moammar Gadhafi, with the help of NATO warships and planes, began in Benghazi. A year later, the city of 1 million remained chaotic, in the grip of heavily armed militias and Islamist militants, some with links to al-Qaida.
The temporary U.S. diplomatic mission, created in hopes of building ties and encouraging stability and democracy, was struck by homemade bombs twice in the spring of 2012. British diplomats, the Red Cross and other Westerners were targeted that spring and summer.
Stevens, based in the capital city of Tripoli, chose to visit Benghazi on the anniversary of the Sept. 11, 2001, attacks, when U.S. embassies around the world were on alert for terrorism.
In Egypt that day, a different sort of trouble struck, and would spread to other Mideast cities over several days: Protesters angry about an anti-Muslim video made in America stormed the U.S. Embassy in Cairo, clambering over the walls and setting flags on fire.
Hours later, the assault in Benghazi began.
A FIERY ASSAULT AND FOUR DEATHS
The Benghazi attack came in three waves, spread over eight hours at two locations. Only in hindsight is the duration of the attack clear because of a lengthy pause before the second assault.
According to accounts from congressional investigators and the State Department's Accountability Review Board:
Around 9:40 p.m. on Sept. 11, a few attackers scaled the wall of the diplomatic post and opened the front gate, allowing dozens of armed men in. Local Libyan security guards fled. A U.S. security officer shepherded Stevens and Sean Smith, a State Department communications specialist, into a fortified "safe room" in the main building.
Attackers set the building and its furniture afire with diesel fuel. Stevens and Smith were overcome by blinding, choking smoke that prevented security officers from reaching them. Libyan civilians found Stevens in the wreckage hours later and took him to a hospital, where he, like Smith, died of smoke inhalation.
He was the first U.S. ambassador to be killed in the line of duty in more than 30 years.