Sri Lanka's police chief and a top defence official have been arrested over their alleged failures to prevent the Easter Sunday bombings that killed more than 250 people, a police spokesperson said.
The bombings on April 21, claimed by the Islamic State of Iraq and the Levant (ISIL or ISIS), took place despite repeated warnings from Indian intelligence that an attack was imminent.
Inspector General Pujith Jayasundara and former defence secretary Hemasiri Fernando were arrested on Tuesday, a day after Attorney General Dappula de Livera said they should be arrested and charged with "grave crimes against humanity" for failing to prevent the attacks on churches and luxury hotels.
Both men were at different hospitals undergoing treatment when detectives arrested them, spokesperson Ruwan Gunasekera said, adding the move followed instructions from de Livera who said they could press murder charges.
They should be brought before a magistrate for their "criminal negligence" de Livera said in a letter to the acting police chief.
"Their negligence amounts to what is known under international law to be grave crimes against humanity," he wrote.
Nine other senior police officers have been named by the attorney general as suspects who should be prosecuted for their role in the security lapses.
Jayasundara and Fernando have testified before a parliamentary inquiry and accused President Maithripala Sirisena of failing to follow established protocols in assessing threats to national security. They have denied all the allegations.
Sri Lankan authorities have admitted that warnings sent by India of an impending attack by a local group, the National Thowheed Jamath (NTJ), were ignored.