Uganda police shot dead five suspects and arrested 21 people Thursday, an official said, as part of an investigation into twin suicide bombings claimed by the Islamic State group that killed four people.

Tuesday’s explosions were the latest in a string of attacks in the East African country, which has tightened security and mounted an investigation into the bombings, with President Yoweri Museveni vowing to defeat “the terrorists”.

The attacks in the capital Kampala occurred within minutes of each other, with two suicide bombers on motorbikes disguised as motorcycle taxi riders commonly known as “boda boda” motorcycle taxi drivers detonating a device near parliament, while a third attacker targeted a checkpoint near the central police station.

Ugandan police said the attacks were the work of “domestic terrorists” linked to the Allied Democratic Forces (ADF), an armed group active in the eastern Democratic Republic of Congo that the United States has linked to IS.

On Thursday, counter-terrorism officers in the west of the country killed “four suspected terrorists in Ntoroko who were crossing back to DRC”, police spokesman Fred Enanga told a press conference.

A fifth man was killed near the capital as he tried to escape arrest, Enanga said, adding that Sheikh Abas Muhamed Kirevu was a local Islamic leader who was “responsible for reawakening the terror cells in Kampala”.