A Sri Lankan police officer patrols out side a mosque in Colombo, Sri Lanka, Wednesday, April 24, 2019. The Islamic State group has claimed responsibility for the Sri Lanka attacks on Easter Sunday and released images that purported to show the attackers. Prime Minister Ranil Wickremesinghe said that investigators were still determining the extent of the bombers' foreign links. (AP Photo/Eranga Jayawardena)

A Sri Lankan police officer patrols out side a mosque in Colombo, Sri Lanka, Wednesday, April 24, 2019. The Islamic State group has claimed responsibility for the Sri Lanka attacks on Easter Sunday and released images that purported to show the attackers. Prime Minister Ranil Wickremesinghe said that investigators were still determining the extent of the bombers' foreign links. (AP Photo/Eranga Jayawardena)

COLOMBO, Sri Lanka (AP) — Sri Lanka’s president shook up the country’s top security establishment after officials failed to act on intelligence reports warning of possible attacks before the Easter bombings that killed over 350 people, his office said Wednesday.

The capital of Colombo, meanwhile, remained rattled by reports that police were continuing to conduct controlled detonations of suspicious items three days after the attacks on churches and luxury hotels, and the U.S. ambassador said that Washington believes “the terrorist plotting is ongoing.”

During a televised speech to the nation Tuesday night, President Maithripala Sirisena said he would change the head of the defense forces within 24 hours, and on Wednesday he asked for the resignations of the defense secretary and national police chief in a dramatic internal shake-up. He did not say who would replace them.

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Sirisena said he had been kept in the dark on the intelligence about the planned attacks and vowed to “take stern action” against officials who failed to share it.

Government leaders have acknowledged that some intelligence units were aware of possible attacks weeks before the bombings that struck three churches and three luxury hotels. The death toll rose Wednesday to 359, with 500 people wounded. Police spokesman Ruwan Gunasekara also said 18 suspects were arrested overnight, raising the total detained to 58.

Sri Lankan authorities have blamed a local extremist group, National Towheed Jamaat, whose leader, alternately named Mohammed Zahran or Zahran Hashmi, became known to Muslim leaders three years ago for his incendiary online speeches. On Wednesday, junior defense minister Ruwan Wijewardene said the attackers had broken away from National Towheed Jamaat and another group, which he identified only as “JMI.”

The Islamic State group has claimed responsibility for the attacks. Authorities remain unsure of the group’s involvement, though authorities are investigating whether foreign militants advised, funded or guided the local bombers.

Wijewardene said many of the suicide bombers were highly educated and came from well-to-do families.

“Their thinking is that Islam can be the only religion in this country,” he told reporters. “They are quite well-educated people,” he said, adding that at least one had a law degree and some may have studied in the U.K. and Australia.

A British security official has confirmed a report that a suicide bomber who is believed to have studied in the U.K. between 2006 and 2007 was Abdul Lathief Jameel Mohamed. The security official, who spoke on condition of anonymity because of the sensitive nature of the investigation, said British intelligence was not watching Mohamed during his stay in the country. His name was first reported by Sky News.

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A team of FBI agents and U.S. military officials were helping in the investigation, said U.S. Ambassador Alaina Teplitz.

She told reporters that “clearly there was some failure in the system,” but said the U.S. had no prior knowledge of a threat before the attacks, the worst violence in the South Asian island nation since its civil war ended a decade ago.

Teplitz called that breakdown in communication among Sri Lankan officials “incredibly tragic.”

The U.S. remains concerned over militants still at large and it believes “the terrorist plotting is ongoing,” Teplitz said, adding that Americans in Sri Lanka should continue to be careful.

Although no more bombs were found Wednesday, Sri Lanka has been on heightened alert since the attacks, with police setting off a series of controlled explosions of suspicious objects. The military has been given sweeping police powers it last used during a devastating civil war that ended in 2009.

Government statements about the attacks have been confused and sometimes contradictory, with Gunasekara, the police spokesman, telling reporters that there were nine suicide bombers — two more than officials said one day earlier.

One of the additional bombers was the wife of another bomber, he said. The woman, two children and three policemen died in an explosion as authorities closed in on her late Sunday, hours after the main attacks were launched. The ninth suicide bomber has not been identified, though two more suspects were killed in a later explosion on the outskirts of Colombo.

Prime Minister Ranil Wickremesinghe edged away from comments made by his state minister of defense that the bombings were carried out in apparent retaliation for the March 15 mosque shootings in Christchurch, New Zealand, that killed 50 people. He told reporters Wednesday that the mosque attack may have been a motivation for the bombings, but that there was no direct evidence of that. An Australian white supremacist was arrested in the Christchurch shootings.

While Sri Lanka’s recent history has been rife with ethnic and sectarian conflict, the Easter bombings still came as a shock to the country of 21 million.

It is dominated by Sinhalese Buddhists but also has a significant Tamil minority, most of whom are Hindu, Muslim or Christian.

Tamil Tiger rebels were known for staging suicide bombings during their 26-year civil war for independence, but religion had little role in that fighting. The Tigers were crushed by the government in 2009. Anti-Muslim bigotry fed by Buddhist nationalists has swept the country since the war ended but Sri Lanka has no history of Islamic militancy. Its small Christian community has seen only scattered incidents of harassment.

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Associated Press writers Bharatha Mallawarachi and Jon Gambrell in Colombo and Gregory Katz in London contributed.