Sri Lankan police arrested three people and seized 21 locally made grenades and six swords during a raid Thursday in Colombo, according to a police spokesperson, as the capital remained jittery over the Easter Sunday attacks that have killed at least 359 people and injured hundreds.

The spokesperson, speaking to Reuters, didn’t provide further details or suggest the raid was linked to the suicide bombings at three hotels and three churches.

But the country’s former navy chief says the father of two of the suicide bombers has been arrested on suspicion of aiding his sons. Jayanath Colombage, now a counterterrorism expert at the Pathfinder Foundation, confirmed the arrest to The Associated Press on Thursday.

ISIS has claimed responsibility for the attacks, which has resulted in the detainment of at least 58 suspects. Authorities remain unsure of the group’s involvement, though they are investigating whether foreign militants advised, funded or guided the local bombers.

On Thursday, authorities in Colombo set off more controlled detonations of suspicious items, soldiers stopped and searched vehicles, and some businesses advised staff to stay indoors.

Overnight, more people, including foreigners, were swept up for questioning as domestic and international authorities investigated deeper into the bombings, believed to have been the deadliest such attack claimed by the Islamic State in Iraq and Syria.

Bomb scare leads to lockdown

Sri Lankan authorities locked down the central bank and shut the road leading to the Colombo’s airport because of a bomb scare on Thursday after a suspicious vehicle was spotted at a car park. The street outside the

building near the World Trade Center was blocked to traffic before the security alert was lifted. And the road to the airport was reopened when the alert was declared a false alarm.

John Keells Holdings, the parent company of the Cinnamon Grand hotel, one of the sites stricken in the Easter bombings, told employees at its various hotel properties to stay inside until at least 2:30 p.m. “further to the communications we have received” in an email shared with The Associated Press.

It was not immediately clear where the warning originated, and a police spokesperson did not respond to several calls and messages.

The streets around Dematagoda, a wealthy Colombo neighbourhood where officials say many of the bombing suspects lived, were quiet Thursday.

Investigators continued to comb through a mansion with nine front balconies where investigators said suspects detonated a ninth bomb Sunday that killed three police officers who were pursuing them. A white BMW was parked outside a garage partially blown out in the blast.

In a house on the other side of a quiet, leafy lane full of mansions, a 14-year-old boy said he used to ride bicycles and play soccer with one of the suspect’s children, a 10-year-old boy who frequently visited his relatives there, and that the other children at the house were too young to play outside. He said his entire house shook when the bomb went off.

Sri Lankan police continued their search for additional explosives, detonating a suspicious item in a garbage dump in Pugoda, about 35 kilometres east of Colombo.

Sri Lanka’s civil aviation authority also banned drones and unmanned aircraft “in view of the existing security situation in the country,” according to a statement.

Kumari Fernando, who lost her husband, Dulip Fernando, and two children, Dulakghi and Vimukthi, during the bombing at St, Sebastian’s Church, yells towards the graves during a mass burial for victims at a cemetery near the church in Negombo on Wednesday. (Thomas Peter/Reuters)

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

Most victims from Sri Lanka

Most of the victims in the Easter Sunday bombings 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.

Father Gregory Silva says Sri Lanka is a nation of love and kindness, and is struggling to cope with the aftermath of Sunday’s bombings 0:18

Japan’s Foreign Ministry has confirmed one Japanese national was killed and four others injured in the Easter bombings. The body of the person who died was returned to Japan early Thursday.

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

Junior Defence Minister Ruwan Wijewardene said at least one had a law degree and others may have studied in the United Kingdom 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.

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 defence secretary and national police chief without saying who would replace them.

Makeshift wooden crucifix’s mark the graves of people killed in the Easter Sunday attack on St. Sebastian’s Church. (Carl Court/Getty Images)

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, Wijewardene said the attackers had broken away from National Towheed Jamaat and another group, which he identified only as JMI.

A Sri Lankan navy soldier searches a truck at a checkpoint in Colombo on Wednesday. (Dinuka Liyanawatte/Reuters)