The death toll in a deadly attack on an upscale Kenyan hotel complex has risen to 21, including 16 Kenyans, one American and three of African descent.

The American, businessman Jason Spindler, was having lunch in the lobby in the DusitD2 hotel when he was gunned down. Jason, who once worked at the World Trade Center, was a survivor of 9/11, his brother said.

The attackers, believed to be members of the insurgent group al-Shabab, claimed responsibility for the 19-hour siege of the hotel and its surrounding office buildings Tuesday, Jan. 15, which they said was a response to the declaration by President Trump of Al-Quds (Jerusalem) as the capital of Israel.

The assault on the hotel and office complex recalled a September 2013 assault by al-Shabab at the Westgate shopping mall, just a mile away. Two years later, masked al-Shabab militants staged an early morning raid on Garissa University College in eastern Kenya, killing at least 147 students. Fifteen hours after the siege began, four gunmen from the Somali group were killed.

“People thought that the security agencies were on top of things, so it really is a surprise to many people,” Emmanual Kisiangani, a Nairobi-based political analyst, said in a press interview. “It’s a very difficult thing to prevent these attacks totally,” he said. “They say that the security agencies always need to be right, but terrorists only need to be right once.”

Al-Shabab’s stated reason for the attack comes as the U.S. ramps up airstrikes against the Somalia-based group. Unmanned U.S. drones conducted 47 strikes in 2018, up from 31 in 2017, according to the U.S. Africa Command. Another recent strike was Jan. 8. Several strikes in December killed 62 al-Shabab fighters.

After the attack on the hotel complex, the U.S. launched an airstrike in Somalia and killed 52 Islamic militants, according to military officials.

More than 300 al-Shabab fighters were killed in last year’s strikes, according to U.S. news reports. Under new rules of engagement enacted by the Trump administration, the U.S. may preemptively strike militants that may not pose an immediate threat to Americans or their allies.

The airstrikes appear to have had little effect on the group’s ability to recruit new members, Lauren Blanchard, an analyst at the U.S. Congressional Research Service, was quoted to say. Forced out of major cities, al-Shabab is said to control large parts of rural southern and central Somalia and continues to carry out high-profile suicide bombings and other attacks in Mogadishu and elsewhere.

Just before the attack took place, Nairobi was being drenched by an unseasonable thunderstorm. The attackers arrived, dressed in black. Witnesses said they fired into offices and threw grenades down stairwells, leaving body parts and blood strewn in hallways.

Kenyan special forces and private security contractors led the building employees to safety.

The following day, Nairobians were seen coming out in large numbers to donate blood. Kenyan President Uhuru Kenyatta declared that all the attackers had been eliminated and 700 civilians evacuated to safety. But up to the time of his announcement, gunshots could be heard from the besieged buildings.

The rise of al-Shabab in Somalia should be seen in the context of decades of mismanagement, dictatorship and abuse, wrote Yohannes Woldemariam, formerly a professor of international relations and environmental studies at Fort Lewis College in Colorado.

They rose to power after a moderate group, the Union of Islamic Courts, was blocked and some 300,000 people displaced. The Horn of Africa country was soon inundated with foreign forces led by Ethiopia and later Uganda and Burundi. Woldermariam’s piece can be read in full on the online newsletter