Indian police stumbled upon the plot for what would become the bloodiest Islamic State group-linked attack in South Asia, but the Sri Lankan authorities they warned didn’t act in time.

“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,” said counter-terrorism expert Ajai Sahni.

Indian police discovered the lapse when they managed to break into the communications of National Towheed Jamaat, the group now believed responsible for the Easter Sunday attacks on churches and hotels which killed more than 250 Christians.

Top Sri Lankan officials acknowledged some of the island nation’s intelligence units were given advance notice about the attacks 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 was kept out of high-level security meetings since Sirisena tried to oust him last fall, said they only learned about the plot after it happened.

Why the warnings went unheeded is the subject of intense public debate.

Separately, 15 people, including six children, were killed Friday when militants linked to the Easter attacks opened fire and set off bombs during a raid by Sri Lankan security forces.

The gun battle came amid widespread fear of more attacks as officials hunt for “sleeper cells,” which may contain Sri Lankans who returned from fighting with ISIS in Syria. Some 80 people have been arrested.

While fear of new attacks reigns, a curfew is in place for the capital, Colombo, and Catholic leaders canceled masses indefinitely. Officials also urged Muslims to stay home for prayers in an extraordinary call by the clergy to curtail worship.

With Post wires