Sri Lanka Is Rattled by New Threats as Officials Argue Over Responsibility

Sri Lankan police officials on Thursday warned of possible new attacks after the deadly Easter Sunday suicide bombings, as Muslim leaders urged the cancellation of Friday Prayer and the cardinal of Colombo, the capital, suspended services for Catholic worshipers through the weekend.

Officials said privately that the authorities were trying to find at least one person believed to be armed with explosives.

A letter distributed to security officials said there was “credible information” that the radical Islamist group National Thowheeth Jama’ath, which the authorities say carried out the Sunday bombings, was planning another attack “specifically targeting Sufi shrines.” Several officials confirmed the authenticity of the letter.

Sufism is a mystical strain of Islam that sometimes comes under attack from more extremist forms of Islam.

The worries about new dangers came as officials debated who was to blame for a series of missed chances to prevent the bombings Sunday at churches and hotels in three Sri Lankan cities, which killed more than 350 people.

President Maithripala Sirisena, who has tried to deflect criticism that he bears some responsibility for the lack of action on intelligence alerts warning of attacks on churches, had called on Wednesday for two security officials, Pujith Jayasundara, the inspector general of the police, and Hemasiri Fernando, the defense secretary, to resign.

A day later, there was no public confirmation they had done so.

Sri Lanka’s government is bitterly divided, which may have contributed to the security failures. Prime Minister Ranil Wickremesinghe, a rival of the president, complained that he had not been allowed to attend security meetings before the attacks and was unaware of the threats.

The fears of fresh attacks swirled after the United States ambassador had warned a day before of possible continuing terrorist plots. Israel’s Counter-Terrorism Bureau on Thursday issued a Level 2 travel advisory, the second highest, warning of a “high concrete threat” in Sri Lanka.

Police roadblocks went up around the country on Thursday, and the Central Bank and office buildings in Colombo were temporarily put on lockdown.

The police briefly closed the road to the country’s main airport, in Negombo, while they searched a suspicious car. A small blast was reported in a town about 20 miles east of Colombo. No one was injured.

The police in Colombo arrested three people with more than 20 grenade-like explosives and six swords, local news media reported.

And Sri Lanka’s civil aviation authority said that it was banning the use of drones because of continuing security concerns.

With warnings of more attacks circulating through Colombo, many people stayed home. The Liberty Plaza mall, one of the city’s biggest, was deserted. Many shops were closed. The few that were open stood empty.

Many people said they felt uneasy.

“I don’t know what to eat, I don’t know when to eat, my whole body is trembling,” said Indika Manamperi, the owner of two restaurants in the food court.

Mr. Manamperi said he used to be a major in the Sri Lankan Army and felt completely demoralized by the terror attacks.

“There was enough of a blood bath for the past 30 years and again our children have to face the same tune?” he said, referring to the country’s long civil war, which ended a decade ago.

What really distressed him, he said, was Sri Lanka’s political dysfunction during such a painful crisis.

“I’m stressed because these Sri Lankan politicians don’t want to take responsibility,” he said. “It’s a pathetic state of affairs.”

The attacks Sunday came after a series of missteps by the authorities.

There was the security memo 10 days before the attacks that outlined the threat of suicide bombings at churches, based on Indian intelligence reports, with names, addresses and phone numbers of suspects. But many officials did not see the memo, and Cardinal Ranjith criticized the government for failing to act.

There was the arrest and subsequent release of at least one of the suicide bombers, which has raised further questions about how the authorities missed the bomb plot.

And there were new warnings from Indian intelligence, including one shortly before the killers set off their powerful bombs in packed churches and hotel restaurants.

Investigators continued to examine what role the Islamic State, which claimed responsibility for the Easter attacks on Tuesday, actually played in them.

The group released a video that showed Mohammed Zaharan, an extremist preacher who was the leader of National Thowheeth Jama’ath, and masked disciples proclaiming their allegiance to the group. Mr. Zaharan is believed to have been one of the suicide bombers.

But National Thowheeth Jama’ath had previously been known for little besides the destruction of the Buddhist statues, raising questions about whether it received foreign assistance and about the extent of ISIS’ involvement. ISIS once held vast territories in Iraq and Syria, and the collapse of its caliphate last month has heightened fears of its fighters spreading violence elsewhere.

The bombers were all Sri Lankan, mostly well educated and from middle-class families, officials said. The eight bombers included two sons of a spice tycoon, Mohammad Yusuf Ibrahim. A woman, described as the likely wife of one of those sons, detonated an explosive that killed herself, her two children and several police officers as the family’s villa was raided Sunday.

On Thursday, officials said that Mr. Ibrahim had been arrested on charges of aiding his sons in the attacks.

The warning of possible attacks against Sufi mosques could tap into decade-old tensions. Threats to local Sufis in the Kattankudy area on the country’s east coast go back as far as 2006, when local Sufis appealed for help after asserting that an influx of extremist Wahhabi Muslims was making life hard for them.