Sri Lanka banned drones and unmanned aircraft and set off more controlled detonations of suspicious items Thursday four days after suicide bombing attacks killed more than 350 people in and around the capital of Colombo.

Sri Lanka’s civil aviation authority said that it was taking the aircraft measure “in view of the existing security situation in the country.”

Hobby drones have been used by militants in the past to carry explosives. Iraqi forces found them difficult to shoot down while driving out the ISIS extremist group, whose members loaded drones with grenades or simple explosives to target government forces. And Yemen’s Houthi rebels has used drones, most recently to target a military parade in January, killing troops.

Sri Lankan police continued their search for explosives, detonating a suspicious item in a garbage dump in Pugoda, about 35 kilometers (22 miles) east of Colombo.

The attacks Easter Sunday mainly at churches and hotels killed at least 359 people and wounded 500 more, the government said Wednesday. Most were Sri Lankan but the Foreign Ministry has confirmed 36 foreigners died. The remains of 13 have been repatriated. Fourteen foreigners are unaccounted for, and 12 were still being treated for injuries in Colombo hospitals.

A top Sri Lankan official has said that many of the suicide bombers were highly educated and came from well-off families.

Junior defense minister Ruwan Wijewardene said at least one had a law degree and others may have studied in the UK and Australia.

Australian Prime Minister Scott Morrison said one of the bombers had been in the country on a student visa with a spouse and child before leaving in 2013.

A British security official also confirmed one bomber is believed to have studied in the UK between 2006 and 2007. The security official, who spoke on condition of anonymity because of the sensitive nature of the investigation, said British intelligence was not watching Abdul Lathief Jameel Mohamed during his stay in the country. His name was first reported by Sky News.

Sri Lankan government leaders have acknowledged that some intelligence units were aware of possible terror attacks against churches or other targets weeks before the bombings. The president asked for the resignations of the defense secretary and national police chief without saying who would replace them.

Sri Lankan authorities have blamed a local extremist group, National Towheed Jamaat, whose leader, alternately named Mohammed Zahran or Zahran Hashmi, became known to Muslim leaders three years ago for his incendiary online speeches.

On Wednesday, junior defense minister Ruwan Wijewardene said the attackers had broken away from National Towheed Jamaat and another group, which he identified only as “JMI.”

The ISIS has claimed responsibility for the attacks. Authorities remain unsure of the group’s involvement, though authorities are investigating whether foreign militants advised, funded or guided the local bombers.

Police spokesman Ruwan Gunasekara has said 58 suspects have been detained since the bombings.