Police said Mohamed Zahran, leader of the National Towheed Jamaat militant group, had been killed in one of the bombings. The group’s second in command was also arrested, police said.
Zahran amassed an online following for his hate-filled sermons. Some were delivered before a banner depicting the Twin Towers.
Australian Prime Minister Scott Morrison said Friday that the attackers responsible for the bombings were supported by the Islamic State group. Around 140 people in Sri Lanka had connections to ISIS, Sri Lankan President Maithripala Sirisena said.
"We will completely control this and create a free and peaceful environment for people to live," he said.
Investigators determined the attackers received military training from someone called "Army Mohideen.” They also received weapons training overseas and at some locations in Sri Lanka, according to authorities.
A copper factory operator arrested in connection with the bombings helped Mohideen make improvised explosive devices, police said. The bombings have led to increased security throughout the island nation as authorities warned of another attack.