COLOMBO, Sri Lanka—While monitoring the usual channels, Indian police stumbled upon something extraordinary: a detailed plot for what would become the bloodiest attack linked to the ISIS group in South Asia.

Police was investigating suspected sympathizers of the withered caliphate in southern India when a name they had no record of surfaced—National Towheed Jamaat, the Sri Lankan ISIS-backed extremist organization that authorities say conducted the coordinated Easter Sunday attacks on churches and hotels in Sri Lanka that killed more than 250 people.

Indian police managed to break into the group’s communications and began tapping into the plot, according to Ajai Sahni, executive director of the Institute for Conflict Management in New Delhi.

“That is why the kind of detailing of the incident they received was very, very specific,” Sahni said. “They knew the group, they knew the targets, they knew the time, they knew the whereabouts of the suicide bombers, and all of this was communicated to the Sri Lankan government.”

Top Sri Lankan officials have acknowledged that some of the island nation’s intelligence units were given advance notice about the attacks—starting weeks ago and up until the morning of the bombings—but that little was done to prevent them.

Both President Maithripala Sirisena, who is also Sri Lanka’s minister of defense and in charge of national police, and Prime Minister Ranil Wickremesinghe, who has been kept out of high-level security meetings since Sirisena tried to oust him last fall, said they only learned about the plot after it had been carried out.

“The fact is, it’s very, very specific information and that has been conveyed to everyone in writing. That is the action that was missing in some cases. That’s what we’re investigating,” Wickremesinghe said.

The first intelligence brief from India arrived April 4, more than two weeks before the bombings. It said a suicide terrorist attack was planned against “some important churches” and listed six people likely to be involved.

The deputy inspector of police shared the report with at least four security unit directors, including those responsible for “VIPs” and foreign embassies, along with a memo, urging the directors to pay extra attention to the places and people in their care.

India’s final intelligence warning came just before the Easter morning blasts, Sahni said.

Why the warnings went unheeded is the subject of intense public debate, with some blaming the dismantling of a system built by former strongman President Mahinda Rajapaksa for rapid response to rebel activity during Sri Lanka’s long civil war.

For 26 years, the Marxist Tamil Tigers organization fought for separatism from the Sri Lankan government. Military forces under Rajapaksa’s brother, then-Secretary of Defense Gotabhaya Rajapaksa, brutally crushed them in 2009.

The current state minister of defense, Ruwan Wijewardene, said “weakness” within Sri Lanka’s security apparatus led to the failure to prevent the Easter bombings.

Sirisena, while campaigning for the 2015 election to defeat Rajapaksa, had stressed the need for fresh investigations of military officials, including intelligence officers accused of abducting and killing civilians, political opponents, and journalists during the civil war.

Since then, some military officials have been arrested on charges related to their actions during the war and remanded in detention facilities. Court cases are ongoing.

But on Friday, Sirisena, perhaps with an eye toward the 2020 election, said that arresting military intelligence officials after the civil war had weakened national security. He promised a shake-up, asking for the resignations of both his secretary of defense and inspector general of police.

Prime Minister Wickremesinghe told local media on April 28, “We’ve had domestic terrorism in the form of Liberation Tigers of Tamil Eelam (LTTE), one of the worst groups. Secondly, what we face now is part of the jihadi international terrorist campaign, so we have to face up to it.

“Here we see a small group of people, unlike the LTTE which was a large group mainly involved in guerrilla and conventional fighting. Explosives and suicide bombers were support groups. On the other hand, this group replies completely on this type of suicide missions. So, we’ve got to adjust to the fact that they may have had some international connections.

“But on the bright side, our reconciliation worked and there was no communal backlash. People have held together, Christians and Muslims have worked together, we have to build on this and eliminate terror.”

Prime Minister of Sri Lanka Ranil Wickremesinghe : India gave us the information and we are thankful, India and Srilanka have been been exchanging information for sometime #TerrorAttack pic.twitter.com/MnWr00TCIO — Doordarshan News (@DDNewsLive) April 27, 2019

He added, “As far as our economy is concerned, we are worried, of course. For tourism, May was the beginning of the lean season, but we have to resolve so that people come in by August, when our tourist season begins. But, of course, our earnings would not be as high as we had anticipated.”

Wickremesinghe said that the authorities would now focus on terrorist sleeper cells.

“Police and security forces are rounding up those involved, but they’re also rounding up the sleepers, those used on second and third rounds (of attacks),” he said. “The danger has come down drastically, (but) we do have to pick up some more sleepers, which we will do in the next few days.”

Some experts believe Sri Lankan security forces may not have given much credence to Indian intelligence before the attacks because of its controversial role in the civil war.

India’s external intelligence group, the Research and Analysis Wing or RAW, initially supported Tamil militants, training and arming the Liberation Tigers of Tamil Eelman in the 1970s. But after the group’s terrorist activities in the 1980s, RAW withdrew its support.

New Delhi made a pact with Colombo in 1987 to send peacekeeping forces to the island on its southern tip, and they ended up fighting the rebels. They were asked to withdraw a few years later amid allegations of abuses against Tamils. In 1991, Indian Prime Minister Rajiv Gandhi was assassinated by a Tamil Tiger suicide bomber.

India questioned Sri Lanka’s heavy-handed approach to defeating the Tigers in the final months of the war, when tens of thousands of civilians were reportedly killed by government troops. Thousands more are still missing. Tamils and other civilian groups in the country’s north and east are still reeling from the effects of the war.

Indian security and intelligence agencies lost some of their “moral authority” with the Sri Lankans, said M.K. Narayanan, the former head of RAW.

“What really happened was India lost moral authority. India did not accept the policies that were being followed, so they lost a lot of support in Sri Lanka,” he said.

Genealogical and cultural ties between Sri Lanka and India date back thousands of years. According to folklore, the island’s majority Sinhalese are descendants of an Indian prince banished there 2,000 years ago.

The nation’s minority Tamils, meanwhile, are in part the descendants of more than a million tea and rubber plantation workers brought to Sri Lanka from southern India by British colonial rulers in the 18th and 19th centuries.

And India shares intelligence with its neighbors in part to keep them within its sphere of influence, Narayanan said.

Located just 14 miles off its southeast coast, India sees Sri Lanka as a bulwark in its military defenses to ward off potential Chinese incursions. Soon after the Easter attacks, India deployed naval and coast guard ships along the narrow Palk Strait.