A senior U.S. military official has confirmed that a military drone attack Monday afternoon killed two top leaders of the al Qaeda-linked terror group that massacred civilians at a Nairobi, Kenya mall last month.

The official said that the attack on a single vehicle in southern Somalia had killed two leaders of al Shabaab, including its most important explosives expert, a man named Anta. The official did not identify the second man killed.

A car carrying the two leaders was struck by Hellfire missiles fired from a Predator, said the official, who contended that no one outside the vehicle was killed.

Earlier, witnesses told al Jazeera said the strike happened near the town of Jilib, about 70 miles north of al-Shabaab's former stronghold of Kismayo near the Kenyan border.

"It was after afternoon prayers between 1:30 p.m. and 2 p.m. when I heard a loud bang. Just one big bang," a witness from Jilib told Al Jazeera. "I came to the scene shortly after. I saw two dead bodies. Then al-Shabaab fighters came to the scene and took the bodies from the Suzuki vehicle. It was a drone strike."

It's the second time in a month that U.S. forces have gone after the senior leadership of al-Shabaab.

On October 7, some 20 U.S. Navy SEALs launched an assault on the town of Barawe along the Indian Ocean shore, hoping to capture or kill Ikrima, in charge of the group's external operations, including the September attack on the Westgate Mall in Nairobi that killed at least 67 people.

The SEALs had to withdraw from the shoreline after being spotted by al-Shabaab fighters and taking heavy fire. U.S. officials told NBC News that the commander on the ground also saw large numbers of women and children near the compound.

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