COLOMBO, Sri Lanka — Five days after coordinated bombings on Easter Sunday killed at least 250 people, this shellshocked tropical nation and its capital remained on edge Thursday, as officials warned of new possible attacks and religious leaders canceled services around the country.

The national health service announced Thursday night that the death toll from Sunday’s blasts, previously said to have reached more than 350, was actually about 100 lower. Health officials said that the discrepancy stemmed from the difficulty of identifying individuals from many body fragments and that an exact figure was still not available. 

The warning of possible additional attacks came from the national police inspector general’s office, which circulated a letter Thursday to other police agencies saying that the same groups that carried out the Easter blasts had threatened to attack mosques where Muslim religious leaders are buried. The letter, a copy of which was obtained by The Washington Post, said the threat should be addressed with urgent measures.

Prime Minister Ranil Wickremesinghe, in an interview with the Associated Press, warned that suspects linked to Sunday’s attacks remain at large and “may go out for a suicide attack.”

The U.S. Embassy in Colombo in a tweet urged people to avoid places of worship because of the risk.

Amid outrage over government failures to act on warnings of possible terrorist attacks around Easter, Defense Secretary Hemasiri Fernando stepped down from his post after Sri Lanka’s president demanded his resignation Wednesday. His departure was the second high-level official change stemming from the bombings. 

The national police inspector general, Pujith Jayasundara, resigned Wednesday.

The government has blamed the attacks on a local Islamist extremist group called National Thowheed Jamaath, which has been active in the eastern region of Kattankudy, home to a large Muslim population. The Islamic State asserted responsibility for the attack, and video emerged this week of the suspected ringleader of the attack pledging allegiance to the Islamic State and its leader, Abu Bakr al-Baghdadi.

Nalaka Sanjeewa Pieris said he helped pull the injured from the St. Sebastian’s church in Negombo, Sri Lanka, after it was bombed on April 21. (Joyce Lee, Patrick Martin, Nuwan Attanayake/The Washington Post)

Security forces in Colombo, the capital, cordoned off and searched the country’s central bank and several office buildings. They also searched the luxurious suburban home of a wealthy Muslim spice trader whose two sons were among the suicide bombers.

Police officials and a former navy chief told media outlets Thursday that the merchant, a prominent business leader named Y.M. Ibrahim, had been arrested on charges of aiding his sons in the attacks. 

 The road to the national airport, about 20 miles from the capital, was briefly blocked while security squads searched a suspicious car, and aviation officials said they were banning the use of drones, which have been used by some militant groups to carry explosives. A small blast was reported in another suburb, but no damage or injuries resulted. 

Police in the capital arrested three people who had grenades and swords, and employees at one luxury hotel that was attacked Sunday were told to remain indoors. As police continued to search for explosives, they blew up a suspicious item found in a garbage dump in Pugoda, a town about 20 miles from Colombo. 

Many shops and restaurants remained closed in Colombo, where a nighttime curfew stayed in effect, and in the nearby seaside city of Negombo, where the deadliest bombing took place. Church officials in Negombo have held about 75 funerals and burials since Monday, but formal religious ceremonies there have now been paused for security reasons. One church official said he had been told that police were still searching for bombing suspects in the area. 

M.H. Abdul Halim, the country’s senior Muslim government official, issued an urgent written appeal Thursday asking all Muslims to “avoid gathering” for congregational prayers on Friday and instead to remain at home and “pray for the peace and security of the motherland.”

Halim, the federal minister of postal services and Muslim affairs, said he was acting “in solidarity” with the Catholic Church and “in protest the barbaric acts” of the bombers.

Many Muslims have expressed fears of being blamed for the bombings and targeted for reprisal attacks, and streets and shops in Muslim communities in both cities have been deserted and on edge. Some people said they were worried about security conditions during Ramadan, the Muslim month of fasting and prayer, which will begin in early May. 

“I can’t go to work, I can’t go to shop, I don’t want to leave the neighborhood. I’m even afraid to go to the mosque,” Mohammad Rahmatullah, 62, who runs a street-paving company, said as he sat disconsolately outside his modest home on a dirt alley in Negombo on Wednesday.

Amantha Perera and Joanna Slater contributed to this report.