Central bank locked down as another 16 people are arrested over Easter Sunday blasts that killed 359 people.

Authorities in Sri Lanka have issued a nationwide alert as more people were arrested overnight in connection with the deadly Easter Sunday bombings that left 359 people dead.

In the capital, Colombo, authorities locked down the central bank and the road leading to the airport was shut briefly following a bomb scare on Thursday.

Reporting from Colombo, Al Jazeera’s Florence Looi said the police have issued a countrywide alert for nine vehicles suspected to have been used in the Sunday bombings.

“The situation remains tense and security has been beefed up, with even naval and air force officers patrolling the streets,” she said.

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Police spokesman Ruwan Gunasekera told Al Jazeera several road blocks had been set up throughout Colombo and suspicious vehicles and individuals were being searched.

“We will conduct raids inside private premises if necessary,” he said, asking for people to cooperate in the search operation.

Al Jazeera’s Minelle Fernandez, also reporting from Colombo, said there were intelligence warnings of potential further attacks and “the authorities don’t want to take too many risks”.

A police spokesman said there was an explosion, so far unexplained, in a town east of the capital but there were no casualties. It was not a controlled detonation like other blasts in recent days and was being investigated, he said.

Church services suspended

All Catholic churches in the country have been instructed to stay closed and suspend services until security improves.

“On the advice of the security forces, we are keeping all churches closed,” a priest told AFP news agency.

Security has been beefed up for Cardinal Malcolm Ranjith and for the Bishop’s House in Colombo, sources told Al Jazeera.

More people, including foreigners, were swept up for questioning overnight as domestic and international authorities probed deeper into the bombings, potentially the deadliest operation claimed by the Islamic State of Iraq and the Levant (ISIL) group.

Sri Lankan soldiers perform security checks on motorists at a road in Colombo [Eranga Jayawardena/AP]

Police said on Thursday another 16 people were detained for questioning overnight, taking the number held since Sunday to at least 76.

A police statement said one of those detained was linked to a “terrorist organisation” but gave no other details.

Al Jazeera’s Fernandez said there were widespread concerns over the state of emergency imposed in the country since the attacks.

“Detention of suspects can be done for three months without the usual legal process kicking in, with law enforcement agencies allowed to apply for its extension by up to a year,” she said.

“Civil society activists and constitutional experts are concerned about the scope of the emergency regulations.”

Groups behind attacks

A picture has slowly emerged of a group of nine well-educated, home-grown suicide bombers, including a woman, who carried out the attacks in the South Asian nation. Police say eight out of the nine suicide bombers have been identified.

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However, authorities have also focused their investigations on international links to a domestic Muslim group, National Thowheed Jamath, believed to be behind the attacks.

ISIL, also known as ISIS, offered no firm information to back up its claim of responsibility. It released a video on Tuesday that showed eight men, all but one with their faces covered, standing under a black ISIS flag and declaring their loyalty to its leader, Abu Bakr al-Baghdadi.

The man whose face could be seen has been identified as Mohamed Zahran, a preacher from the east of Sri Lanka known for his hardline views who officials believe was the attack’s mastermind.

Australian Prime Minister Scott Morrison said on Thursday another of the bombers had lived in Australia with his wife and child on a student visa but left in 2013.

Morrison did not identify the man, although his family has said his name was Abdul Latheef Mohamed Jameel.

Crosses cast shadows at the site of a mass burial in Negombo [Athit Perawongmetha/Reuters]

Foreigners detained

Police also said they detained an Egyptian who was found not to have a valid visa or passport. The man taught Arabic in a school about 70km from Colombo and had been living in Sri Lanka for more than seven years.

A police spokesman also said a group of Pakistanis had been detained among an unspecified number of foreign nationals for overstaying their visas.

The bombings shattered the relative calm that has existed in Buddhist-majority Sri Lanka since a civil war against mostly Hindu, ethnic Tamil separatists ended 10 years ago and raised fears of a return to sectarian violence.

Sri Lanka’s 22 million people include minority Christians, Muslims and Hindus. Until now, Christians had largely managed to avoid the worst of the island’s conflict and communal tensions.

President Maithripala Sirisena will meet representatives of different faiths later on Thursday to address concerns of a sectarian backlash.

Meanwhile, Muslims have fled the Negombo region on Sri Lanka’s west coast since scores of worshippers were killed in the bombing of St Sebastian’s church there on Sunday. Communal tensions have since flared.

Hundreds of Pakistani Muslims left the port city on Wednesday, crammed into buses, after threats of revenge.

“Because of the bomb blasts and explosions that have taken place here, the local Sri Lankan people have attacked our houses,” Adnan Ali, a Pakistani Muslim, told Reuters as he prepared to board a bus.

Sri Lanka’s State Minister of Defence Ruwan Wijewardene has said he believed the bombings were carried out in retaliation for the March 15 attack on two mosques in New Zealand without providing evidence or explaining where the information came from.

New Zealand Prime Minister Jacinda Ardern has said she has seen no evidence to support that claim.

Most of the Easter Sunday victims were Sri Lankans, although authorities confirmed at least 38 foreigners were also killed. These included British, American, Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals.

– Additional reporting by Dilrukshi Handunnetti from Colombo