The U.S. State Department is offering a $27 million bounty for information on the whereabouts of six key leaders of al-Shabab, a Somalia-based terrorist organization that has killed thousands of people in the Horn of Africa region. U.S. officials announced Wednesday rewards of as much as $6 million for information leading to the identification or location of Abu Ubaidah, as much as $5 million each for information on Mahad Karate, Ma’alim Daud and Hassan Afgooye, and a possible $3 million each for information on Maalim Salman and Ahmed Iman Ali.

Abu Ubaidah, also known as Direye, took charge of al-Shabab in September 2014, after the death of former leader Ahmed Abdi aw-Mohamed, also known as Godane, who was killed by a U.S. drone. Since then, the al Qaeda-affiliated group has orchestrated dozens of attacks in Somalia and neighboring countries.

Mahad Karate, also known as Abdirahman Mohamed Warsme, played a key role in the April attack on Garrissa University College in Kenya, during which at least 147 people were killed, the State Departmentsaid. Hundreds of students, mostly Christian, were taken hostage by the Islamic militants.

Ma’alim Daud is in charge of al-Shabab’s planning, recruitment, training and operations against the government of Somalia and Western targets, the State Department said. The militant group aims to overthrow Somalia’s western-backed government and impose its strict version of Islamic law.

Hassan Afgooye oversees the group’s complex financial network, which funds attacks like the deadly siege at Garissa University College, the State Department said. Al-Shabab has moved from one illegal business to another, with activities in illicit ivory, smuggled charcoal, kidnapping, piracy ransom, racketeering and even fake charity drives, the New York Times reported.

Maalim Salman leads al-Shabab’s African foreign fighters and has been involved in operations in Africa targeting tourists, entertainment establishments and churches. Meanwhile, Ahmed Iman Ali, a Kenyan, has recruited youth from his home country as well as raised funds for al-Shabab, the State Department said. Ali is apparently the former chairman of Muslim Youth Center in Nairobi, Kenya, and is also al-Shabab’s de facto leader of Kenyan fighters in Somalia, Kenya’s Daily Nation newspaper reported. Ali reportedly was among the masterminds behind al-Shabab’s 2013 attack on the Westgate shopping mall in Nairobi that left more than 60 people dead.

Al-Shabab lost control of Somalia’s capital, Mogadishu, in 2011, but the group has remained a potent threat in the Horn of Africa country. Most recently, the militants bombed a hotel in Mogadishu Nov. 1, killing at least 13 people. In September, the group took control of Buqda, a populous town in central Somalia, as well as two towns in the lower Shabelle region, Reuters reported.