Nairobi: The "terror mastermind" of a brutal hotel attack in Kenya four years ago, according to the United States, is wanted for questioning. The country announced on Thursday that it would pay up to $10 million as a reward. It claimed to be looking for information on Mohamoud Abdi Aden, who it identified as the leader of the jihadist organisation Al-Shabab based in Somalia and responsible for several deadly attacks in Kenya's neighbour. The nearly 20-hour long siege on the upscale DusitD2 hotel compound in Nairobi, Kenya, on January 15, 2019, was blamed on an Al-Qaeda affiliate group. Also Read: Greek court dismisses allegations of spying against migrant rescuers A US citizen was among the at least 21 fatalities, and numerous others suffered injuries. At the time, Kenya claimed that all of the attackers had been defeated. According to Meg Whitman, the US ambassador to Kenya, "Mohamoud Abdi Aden, an Al-Shabab leader, was part of the cell that the planned the DusitD2 hotel attack," she told reporters in Nairobi. According to her, the US was offering a reward of up to $10 million for information that would result in the capture of Aden, who the embassy identified as a Kenyan national, and other people suspected of being involved in the hotel siege. Also Read: Top Brazilian court approves investigation into Bolsonaro for riot Amin Mohamed Ibrahim, the director general of Kenya's Directorate of Criminal Investigations, identified Aden as the "terror mastermind" responsible for the bloodshed. In October of last year, the State Department designated Aden as a "specially designated global terrorist." Since sending its army into Somalia in October 2011 to combat the Islamist militant group, Al-Shabab has repeatedly targeted Kenya. Al-Shabab held the Westgate shopping centre in Nairobi hostage for four days in 2013, resulting in 67 fatalities. 148 people were killed in a 2015 attack at Garissa University in eastern Kenya, almost all of them students. After being identified as Christians, many of them were shot dead in the head. Also Read: JP Morgan: A famous student aid startup bought for $175 Most of the users turned out to be fake The only bloodier attack in Kenyan history, which claimed 213 lives in 1998, was an Al-Qaeda bombing of the US embassy in Nairobi. Since 2008, the US has classified Al-Shabab as a terrorist organisation due to its 15-year bloody insurgency against the weak central government of Somalia. Washington announced in November that it would increase its reward for important Al-Shabab figures, including "emir" Ahmed Diriye, to up to $10 million per individual.