Colombo -- Sri Lanka's Catholic churches suspended all public services over security fears on Thursday and Muslims were urged to avoid gathering for Friday prayers. Both religious groups were still nervous as thousands of troops joined the hunt for suspects in the Easter Sunday bombings that killed 253 people -- a revised death toll the health ministry in Sri Lanka released on Thursday. Authorities previously said at least 359 people were killed.

Security forces made fresh arrests overnight linked to the attacks and seized grenades, swords and other weapons in the capital Colombo as the government faced fast-mounting pressure over its failure to act on intelligence warnings about possible bombings.

The country's prime minister gave a dire warning that more bloodshed was possible as some of the suspects were still believed to be at large, and my still have explosives.

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The U.S. Embassy in Colombo echoed the warning in a tweet, saying "Sri Lankan authorities are reporting that additional attacks may occur targeting places of worship. Avoid these areas over the weekend, starting tomorrow, April 26th through Sunday, April 28th. Continue to remain vigilant and avoid large crowds."

Police asked the public for help in apprehending at least two more young suspects, meanwhile, tweeting photos on Thursday of at least one man and woman. The police did not say how they believed the individuals might be linked to the attacks.

Recriminations have flown since suicide bombers blew themselves up in luxury hotels and churches packed with Easter worshippers. The president asked the police chief and top defense ministry bureaucrat to step down, and on Thursday Defence Secretary Hemasiri Fernando did resign.

Overnight, security forces using newly granted powers under the country's state of emergency arrested 16 more suspects in connection with the attacks. Police have so far arrested 75 people, but tensions remain high with several suspects unaccounted for -- and warnings that they could try to attack again.

Israel asked its citizens to leave Sri Lanka on Thursday, citing what it called an "elevated concrete threat" in the country.

Faithful warned to stay home

A senior Catholic priest told AFP that all public services were being suspended and all churches closed "on the advice of security forces."

Private services for burials would still be carried out, but no public masses were scheduled.

Meanwhile the government minister for Muslim religious affairs, Abdul Haleem Mohamed Hasheem, asked the country's larger Islamic population to avoid mosques on Friday, the Muslim holy day.

"As a sign of solidarity with the Catholic Community and a show of protest against the barbaric act of the ruthless terrorists I appeal to my Muslim brothers to avoid gathering for congregational prayers tomorrow (Friday), but to pray in the homes instead, for peace and security of our Motherland," he said in a letter distributed via Muslim community groups.

A soldier stands guard at St. Anthony's Shrine during heavy rain, days after a string of suicide bomb attacks on churches and luxury hotels across the island on Easter Sunday, in Colombo, Sri Lanka, April 25, 2019. REUTERS

"Major lapse in the sharing of information"

The government has been on the defensive over revelations that specific warnings about an attack went ignored.

Sri Lanka's police chief warned on April 11 that suicide bombings against "prominent churches" by the local Islamist group National Thowheeth Jama'ath (NTJ) were possible, citing information from a foreign intelligence agency.

The Indian intelligence services passed on information in the weeks before the attacks, officials have confirmed to CBS News, specifically about a potentially terror plot. But that information was not shared with the prime minister or other top ministers, the government has said.

"It was a major lapse in the sharing of information," deputy defense minister Ruwan Wijewardene conceded at a press conference on Wednesday.

President Maithripala Sirisena, who is also defense and law and order minister, pledged Tuesday to make "major changes in the leadership of the security forces in the next 24 hours".

On Thursday, he met the country's political parties to discuss the crisis, and later will meet religious leaders as concerns rise about a potential backlash against Sri Lanka's Muslim minority.

Spice tycoon among those questioned

Investigators are still piecing information together about the attack and those involved, with Wijewardene revealing Wednesday that one attacker had studied in Britain and did post-graduate studies in Australia before returning to Sri Lanka.

"Most of them are well-educated and come from middle, upper-middle class families, so they are financially quite independent and their families are quite stable financially, that is a worrying factor in this," the minister added.

One of the people in police custody is Mohamed Ibrahim, a wealthy, well-connected Sri Lankan spice mogul. Two of his sons have been identified as bombers who blew themselves up in Colombo hotels on Sunday. A third son was detained along with the father.

Hilmy Ahmed, the Vice President of the Muslim Council in Sri Lanka, told CBS News senior foreign correspondent Elizabeth Palmer on Wednesday that Ibrahim had a heart attack as he was taken into custody.

The role of ISIS

An FBI team is now in Sri Lanka, Wijewardene said, and Britain, Australia and the United Arab Emirates have also offered intelligence help.

Experts say the bombings bear many of the hallmarks of ISIS attacks, and the government has suggested local militants could not have acted alone. But it has not yet officially confirmed any ISIS role in the blasts.

Speaking on Wednesday to former CIA deputy director Michael Morell, now a CBS News terrorism and security consultant, for the "Intelligence Matters" podcast, Secretary of State Mike Pompeo said it was likely the attack was at least "inspired by ISIS."

A photo released by ISIS' propaganda wing Amaq on April 23, 2019, shows eight men purported by the terror group to be the suicide bombers who carried out the coordinated attacks on three mosques and four hotels in Sri Lanka on Easter Sunday.

A key suspect in the attacks is still unaccounted for: Zahran Hashim, a radical Muslim preacher and leader of a local extremist group known as the NJT.

He appears to be among eight people seen in a video released Tuesday by ISIS after it claimed the attacks, seen leading a pledge of allegiance to ISIS chief Abu Bakr al-Baghdadi. In the photo above, he is seen in the center of the group -- the only one without his face covered.

Officials said it was still unclear whether Hashim was among the suicide attackers or had escaped after the blasts. A man strongly resembling him is seen in security camera video from one of the hotels on the day it was attacked.

Dozens of foreigners were among the victims of the blasts, including three children of a Danish billionaire, eight Britons, 10 Indians and four Americans.

The United Nations said at least 45 children, Sri Lankans and foreigners, were among the dead.