The father of two of the suspected Easter suicide bombers was arrested Thursday on suspicion of aiding his sons, Sri Lanka's prime minister said, as businesses and religious leaders worried about a new attack four days after blasts killed more than 350 people.

Across Colombo and in Negombo, the seaside city north of the capital where a bombing killed 110 people, residents and office workers stayed indoors and closed up shops, leaving both cities unusually quiet.

Prime Minister Ranil Wickremesinghe told The Associated Press that there were still suspects at large and they may have explosives. Police, meanwhile, issued a public appeal for three women and one man suspected of involvement in the Islamic State group-claimed attack.

Former navy chief Jayanath Colombage, now a counterterrorism expert, said it was unclear whether the father or his home, where an explosion Sunday killed three police officers, had been under surveillance ahead of the attacks. Wickremesinghe identified him as a wealthy spice trader.

Sri Lankan police spokesperson Ruwan Gunasekera did not answer repeated calls and messages seeking comment, but earlier said 58 people had been detained since the bombings.

The Rev. Niroshan Perera, a priest overseeing funerals of people killed in the Negombo blast at St. Sebastian's church, said Catholic churches in the city, known as "Little Rome" for its many religious buildings, were all closed and had canceled Mass upon the advice of government security officials.

Perera said an official had warned him that police were still searching for two armed suspects.

"Little bit, we are nervous," he said.

In Colombo, John Keells Holdings, the parent company of the Cinnamon Grand hotel, one of the sites targeted in the Easter Sunday bombings, told employees at its various hotel properties to stay inside "further to the communications we have received" in an email shared with the AP.

Few people were out in Dematagoda, a wealthy Colombo neighborhood where investigators combed through the home of the arrested father of the suspects, a mansion with nine front balconies where investigators said suspects detonated a ninth bomb on Sunday that killed three police officers who were pursuing them. A white BMW was parked outside a garage partially blown out in the blast.

Officials say many of the bombing suspects were highly educated and came from well-off families who lived in the neighborhood.

In a house on the other side of a quiet, leafy lane full of mansions, Buhari Mohammed Anwar, 77, a retired primary school teacher, said his neighbor was a nice person who helped the poor.

Of the suspected suicide bombers, he said, "Their father ... didn't expect this. Their father advises them every day. But they don't listen. Children became like that, they don't listen."

Anwar's 14-year-old grandson 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.

Elsewhere, Sri Lankan police continued their search for additional explosives, detonating a suspicious item in a garbage dump in Pugoda, about 22 miles east of the capital.

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.

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 Islamic State group, whose members loaded drones with grenades or simple explosives to target government forces. Yemen's Houthi rebels also have used drones, most recently to target a military parade in January, killing troops.

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.

Junior Defense Minister Ruwan Wijewardene has said that at least one of the suicide bombers had a law degree and others may have studied in the U.K. 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 U.K. 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 the man during his stay in the country.

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

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