By Sanjeev Miglani and Joe Brock

COLOMBO (Reuters) – The scale and sophistication of the Easter Sunday attacks on churches and hotels in Sri Lanka suggested the involvement of an external group such as Islamic State, the U.S. ambassador said on Wednesday, as the death toll from the bombings rose to 359.

The Islamic State militant group claimed responsibility for the coordinated attacks on three churches and four hotels, and details have begun to emerge of a band of nine, well-educated suicide bombers, including a woman, from well-to-do families.

Sri Lankan officials have blamed two domestic Islamist groups with suspected ties to Islamic State.

“If you look at the scale of the attacks, the level of coordination, the sophistication of them, it’s not implausible to think there are foreign linkages,” the U.S. ambassador to Sri Lanka, Alaina Teplitz, told reporters in Colombo.

“Exploring potential linkages is going to be part of (investigations),” she said.

The U.S. Federal Bureau of Investigation and military were supporting the investigation, she said. Britain was also sending a team to help, Sri Lanka said.

Teplitz’s comments came as Sri Lanka’s junior defense minister, Ruwan Wijewardene, conceded that there had been a significant intelligence failure before the attacks, with reports of warnings of strikes not acted on and feuds at the highest levels of government.

“It is a major lapse in the sharing of intelligence information,” Wijewardene told a separate news conference. “We have to take responsibility.”

President Maithripala Sirisena had asked the police chief and defense secretary to quit, two sources close to the president said.

MISSED WARNINGS

Lakshman Kiriella, the leader of parliament, said senior officials had deliberately withheld intelligence about possible attacks.

“Some top intelligence officials hid the intelligence information purposefully … the top brass security officials did not take appropriate actions,” Kiriella, who is also minister of public enterprise, told parliament.

He said information about possible suicide attacks was received from Indian intelligence on April 4 and a Security Council meeting was chaired by President Maithripala Sirisena three days later, but it was not shared more widely.

Police said the death toll had risen overnight to 359 from 321, making it the deadliest such attack in South Asian history. About 500 people were wounded.

Islamic State also released a video late on Tuesday through its AMAQ news agency, showing eight men, all but one with their faces covered, standing under a black Islamic State flag, declaring loyalty to its leader, Abu Bakr Al-Baghdadi.

The one man in the video with his face uncovered was Mohamed Zahran, a Sri Lankan preacher known for militant views.

If the Islamic State connection is confirmed, it would be the deadliest ever such overseas attack linked to the group.

LAW DEGREE

The early Sunday 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.

While the Islamic State video showed eight men, Wijewardene said there were nine suicide bombers. Eight had been identified and one of them was a woman, he said.

“Most of the bombers are well-educated, come from economically strong families. Some of them went abroad for studies,” Wijewardene said.

“One of them we know went to the UK, then went to Australia for a law degree. Foreign partners, including the UK, are helping us with those investigations.”

Wijewardene told parliament on Tuesday that two Sri Lankan Islamist groups – the National Thawheed Jama’ut and Jammiyathul Millathu Ibrahim – were responsible for the blasts. He said on Wednesday the leader of one of those groups blew himself up in the attack on the luxury Shangri-La Hotel in Colombo.

A total of 60 people had been detained for questioning across Colombo since Sunday, Wijewardene said. That total includes a Syrian, according to security sources.

Police searched more homes overnight, leading to the detention of 18 more people.

The overnight raids included areas near the Gothic-style St Sebastian church in Negombo, north of the capital, where scores were killed on Sunday, a police spokesman said.

An unspecified number of people were detained in western Sri Lanka, the scene of anti-Muslim riots in 2014.

“Search operations are going on everywhere, there is tight checking of Muslim areas,” a security source said.

Most of those killed and wounded were Sri Lankans, although officials said 38 foreigners were also killed. That included British, U.S., Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals. Forty-five children were among the dead.

The government has imposed emergency rule and an overnight curfew. It said it has also blocked online messaging services to stop the spread of inflammatory rumours that it feared could incite communal clashes.

(GRAPHIC: Sri Lanka bombings – https://tmsnrt.rs/2Xy02BA)

(GRAPHIC: A decade of peace shattered – https://tmsnrt.rs/2W4wZoU)

(Reporting by Sanjeev Miglani; Editing by Paul Tait, Robert Birsel and Alex Richardson)