Sri Lanka Calls Bombers ‘Well Educated’ and Warns of Ongoing Threat

COLOMBO, Sri Lanka — Nine suicide bombers from mostly educated, middle-class backgrounds carried out the attacks in Sri Lanka that killed more than 350 people on Easter Sunday, the authorities said on Wednesday as they warned of an ongoing terrorist threat and continued making arrests.

The bombers, one of whom was a woman, were all Sri Lankan, officials said. But the authorities were continuing to investigate whether the Islamic State, which on Tuesday claimed responsibility for the coordinated blasts, had provided more than symbolic support, such as by training the attackers or building the bombs.

The authorities said the number of people arrested had risen to 60, and that other individuals involved in the attacks remained at large. As the F.B.I. arrived to assist in the investigation, the American ambassador, Alaina Teplitz, said there were believed to be “ongoing terrorist plots,” a point that Sri Lanka’s state minister of defense also made at a news conference.

“There could be still a few people out there,” said the minister, Ruwan Wijewardene. “Right now, we are asking people to be vigilant. Within the next few days, we will have the situation totally under control.”

Mr. Wijewardene said the leader of the attacks was believed to have been among the suicide bombers, one of whom was a woman. He did not name any of the bombers, and he did not specify whether the leader among them was Mohammed Zaharan, the head of an obscure Islamist extremist group that the authorities have said was behind the attacks.

“They’re quite well educated people,” Mr. Wijewardene said of the attackers. “We believe that one of the suicide bombers studied in the U.K. and then later on did his post-graduate in Australia before coming back to settle in Sri Lanka.” He said the bombers were from different parts of Sri Lanka, but he did not elaborate.

Officials said they were still trying to determine whether the attackers had links to the Islamic State. The terror group, also known as ISIS or ISIL, has released a video showing Mr. Zaharan leading masked, black-clad disciples as they pledged allegiance to the organization.

The Islamic State has not provided any further proof for its claim of responsibility, and Mr. Wijewardene said investigators were trying to determine whether the group had provided training or financing for the attacks. He said they had found no evidence to suggest that the bombers had traveled to the Middle East to fight for ISIS.

The bombings Sunday occurred nearly simultaneously at three churches and three hotels. In the last couple of days, security near the bomb sites has tightened. Schools have been shut until Monday, and the postal department is requiring that items sent by mail be wrapped in front of workers at post offices.

The police said they found a “suspicious bag” at a restaurant in the city of Negombo, near St. Sebastian’s Church, where around 100 people were killed on Sunday. The bag was destroyed on Wednesday in a controlled explosion.

Many mourners on Wednesday focused their anger on the government and the security forces, as grief morphed into rage. All morning long, people gathered near St. Sebastian’s Church in Negombo to mourn the deceased at a mass burial.

One distraught woman could not stop crying and shouting at the police. She blamed them for not acting on prior intelligence warning of the attacks.

An Indian security official, who spoke on the condition of anonymity, said India had interrogated a man last year who was linked with ISIS, and who said he was inspired by Mr. Zaharan’s videos on social media. That intelligence prompted an investigation into Mr. Zaharan, and it was part of the context for an April 11 warning sent to the Sri Lankan authorities about the possibility of church bombings.

The warning was never relayed to church officials, and the Sri Lankan authorities apparently took no action against members of Mr. Zaharan’s group, despite specific information provided by the Indians.

During a national address on Tuesday, President Maithripala Sirisena tried to deflect criticism that he was at least partly responsible for the security failure. He acknowledged that “there was an intelligence report about the attack” but said he was “not kept informed” about it by his subordinates.

Shiral Lakthilaka, his senior adviser, said the security lapse would result in firings. “Positions of secretary of the Ministry of Defense and inspector general of police are positions earmarked for dismissal,” he said.

But many lawmakers dismissed assertions that the president would not have known about the threat memo, saying that blame for the security lapse should go all the way to the top.

Sarath Fonseka, a member of Parliament who was an army chief in the last stage of Sri Lanka’s civil war, told Parliament on Wednesday that he knew about the memo, as did the national intelligence chief. He said it was “obvious that the letter would have gone to the president.”