The U.S. attack, the deadliest against al-Shabab in more than a decade, involved both manned and unmanned aircraft, according to a senior defense official who spoke on the condition of anonymity to speak frankly about the operation. There were believed to be no civilian casualties in the strike, although the Pentagon is still assessing the situation, said the official.

The strike on the camp in Raso, approximately 120 miles north of Mogadishu, is the second U.S. attack on a major terrorist training facility in less than a month. In February, a U.S. airstrike targeted an Islamic State training camp in Sabratha, Libya. A senior leader of the group, Noureddine Chouchane, and 48 other Islamic State fighters were killed.

According to the defense official, U.S. intelligence assets had been watching the camp in Somalia for several weeks prior to the strike. The site was home to a large group of fighters who were scheduled to depart in the coming days.

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Last month, al-Shabab claimed responsibility for a suicide bomber attack that ripped a hole in the side of an Emirati airliner, forcing the plane to land in Mogadishu. No one was killed except the bomber. A statement from al-Shabab claimed the attack was aimed to target western and Turkish intelligence officials, and it is thought that the bomber might have been aboard the wrong plane.

“The removal of these fighters degrades al-Shabab’s ability to meet the group’s objectives in Somalia, including recruiting new members, establishing bases, and planning attacks on U.S. and AMISOM forces,” said Pentagon spokesman Peter Cook, using an acronym for the African Union’s troops.

Along with the attack on the airliner, al-Shabab claimed last month to have ambushed and killed more than 100 Kenyan troops operating in Somalia. While disputed by the Kenyan army, the attack, if true, would be one of al-Shabab’s deadliest to date. Kenya currently has approximately 4,000 troops in Somalia in a bid to support African Union forces fighting in the region.

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In September 2014, a series of airstrikes killed one of al-Shabab’s founders, Ahmed Abdi Godane, in an attack the Pentagon said decapitated the group’s senior leadership.

Though the United States has targeted al-Shabab since 2008, the group has remained resilient and locally active despite losing some of its senior leadership.

Four years ago, al-Shabab, which means “the youth” in Arabic, pledged allegiance to al-Qaeda. More recently, there has been tension with the Islamic State, which has become a rival militant group. In 2015, al-Shabab was targeted by the Islamic State in a propaganda campaign that aimed to convince the group’s leadership to pledge allegiance to Abu Bakr al-Baghdadi, the head of the Islamic State. Despite some infighting, the group remained mostly insulated from the Islamic State’s growing influence.

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Meanwhile, the Pentagon has continued trying to stamp out al-Shabab.

According to data compiled by the think tank New America, there have been 15 Special Operations raids and 12 drone strikes in Somalia since 2003. Prior to 2008, strikes in Somalia were relegated to attacking al-Qaeda forces there. In 2013, U.S. Navy SEALs carried out a nighttime raid in a bid to kill or capture one of al-Shabab’s senior leaders. The raid turned into a prolonged gun battle after the SEALs were discovered, forcing the commandos to retreat to the sea.

In the 13 years of U.S. operations in Somalia, the combination of raids and drone strikes has killed approximately 150 al-Shabab and al-Qaeda militants. All of the drone strikes have occurred during President Obama’s tenure as his administration has sought to contain al-Shabab’s growth.

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In 2013, a number of al-Shabab militia members seized control of the Westgate Mall in Nairobi in an attack that killed at least 67 people. Last year, the group was responsible for an attack on dormitories at Garissa University College in northeastern Kenya that killed 148 people.

There is a small detachment of approximately 50 U.S. advisers in Somalia aiding the African Union troops stationed in the war-torn country. Their deployment in 2013 marked the first time U.S. ground troops were stationed in the country since a detachment of Rangers and Delta Force soldiers withdrew following the failure of Operation Gothic Serpent, known to many as the Black Hawk Down incident, in 1993. The drones that carried out the strike are likely to have been flown from the small U.S. drone base at Chabelley Airfield in nearby Djibouti.

This post has been updated to reflect statements from the Pentagon.