Colombo, Sri Lanka -- The confirmed death toll from the Easter Sunday terror attack in Sri Lanka has reached 359. More security camera video emerged on Wednesday, showing two of the suspected attackers inside a luxury hotel.

Officials have said there were nine bombers in all, including one woman. CBS News senior foreign correspondent Elizabeth Palmer said a video released by ISIS, claiming responsibility for the attack, shows eight of the purported suicide attackers swearing allegiance to the terror group.

But officials are still unclear how much of a role ISIS actually played, beyond inspiring the attack. Sri Lanka's junior defense minister reiterated on Wednesday the government's stance that two relatively unknown local Islamic extremist groups were behind the attack.

Junior minister Ruwan Wijewardene also told reporters that many of the bombers were highly educated and came from well-off families.

"Their thinking is that Islam can be the only religion in this country," Wijewardene said. "They are quite well-educated people," he said, noting that at least one had a degree in law and that some of them may have lived and studied in the United Kingdom or Australia.

Missed warnings

But the man in who stood in the middle of the group shown in the ISIS video -- the only one not hiding his face -- was Moulavi Zahran Hashem, a local Islamist hate preacher who was already known to Sri Lankan intelligence agencies.

"I don't think they took this threat seriously enough, to put enough man power to track him down," Hilmy Ahmed, Vice President of Sri Lanka's Muslim Association, said Wednesday. He said his organization warned security officials three years ago about Hashem.

And it gets worse; immediately before the Easter attacks, there were more warnings.

"We were told that ten minutes before the blast, Indian intelligence had said that this is going to happen," Ahmed told CBS News. "The churches could have been evacuated and at least the number of casualties could have been reduced."

A U.S. official familiar with the intelligence on Sri Lanka has confirmed to CBS News that the government of India did share specific information with Sri Lanka in the days before the attack. The official said the bombings are believed at this time to be ISIS inspired, but officials continue to look for clear links. Evidence was still being gathered and analyzed, with help from FBI agents on the ground.

"The intelligence had been far too complacent," Ahmed said. He agreed with Palmer that it seemed like Sri Lanka's intelligence and security services were more than complacent -- even criminally negligent.

Former CIA deputy director Michael Morell, now CBS News security consultant, says the intelligence failure shows the extent to which Sri Lanka's state security forces are "inept" given the threat they're facing.

"We're fearing a backlash"

All day on Tuesday, Christians buried the victims of the attacks. They were still struggling to absorb the loss of loved ones, and the scale of the atrocity.

But Sri Lankan Muslims know that all the anguish, and the anger with the government's failings, could easily turn ugly.

"We're fearing a backlash," Ahmed told CBS News. "Last night most of us could not go to sleep, because, after 104 people were buried at the same time, you know, emotions can run high."

As the scale of intelligence failure becomes obvious, the government is struggling to explain its actions, and inaction, and to reassure a nervous population.

President Maithripala Sirisena has said he's going to fire all the country's defense chiefs, and restructure the security services. But for many in the country, it will be too little, far too late.