Islamic State has released video of the Sri Lankan Easter Sunday suicide bombers swearing allegiance to militant group's leader Abu Bakr al-Baghdadi.
Bombers had struck churches and hotels with foreign guests from “crusader” countries fighting against the militants, a statement said, and referred to Easter as an “infidel” holiday.
The video and a photograph showed Zaharan Hashmi, the extremist preacher who led the obscure Sri Lankan group accused of carrying out the simultaneous blasts at six targets.
The Islamic State claim on its Aamaq news agency backed Sri Lankan government assertions the attackers had international terrorist help and came as more details of security blunders prior to the bombing were disclosed.
Indian intelligence officials issued detailed warnings to Sri Lankan authorities that Hashmi and his brother were plotting attacks two weeks earlier and even named a suspected bombmaker. Delhi repeated its alert only hours before the bombs went off.
The Sri Lankan Muslim community had also repeatedly told the government over the past three years that Hashmi had been using online sermons to preach hate against Buddhists, Hindus and Christians.
As the scale of the missed warnings became clear opposition parties blamed the government and the prime minister promised cabinet members would be sacked.
A team of British intelligence officers, from MI5 and MI6, have flown out to Sri Lanka to assist with the investigation along with Metropolitan Police counter-terrorism officers.
The agents are urgently seeking any possible links between the bombers and the UK as a priority. Holiday makers with video of the blasts are being urged to contact Scotland Yard.
Intelligence sources are puzzled how Hashmi's obscure local group, previously implicated in desecrating Buddhist religious sites, has taken the "massive leap" to carrying out Sunday's atrocity.
Sri Lankan authorities are also accused of ignoring the homegrown threat from jihadists, even though some 30-odd Sri Lankan citizens had joined Islamic State in Iraq and Levant (Isil) from 2015 onwards.
Chilling details of the attack continued to emerge including CCTV footage of one young bomber calmly stopping to pat a child's head seconds before he strode into a church and detonated.
India’s Research and Analysis Wing (RAW), the country’s external security agency, had issued precise warnings only 10 days before the bloodshed.
Hashmi, leader of the fringe National Thowheed Jamath (NTJ) group, was planning to suicide bomb Catholic churches and the Indian High Commission in Colombo, the warning said.
The warning also named another Hashmi associate, Badrudeen Mohammad Mohiudeen, a former Sri Lankan Army bomb expert, who may have helped make the bombs.
NTJ had until this week only been known for links to vandalism attacks on statues in Buddhist shrines carried out late last year. But it has now emerged members of the group were arrested in January after 100 kg of military grade explosives and detonators were seized at a remote plantation in Wanathawilluwa on the island's west coast. All four were released on bail pending trial.
The farm appeared to have been a training ground for jihadists who were reportedly aiming to blow up Buddhist monuments in the ancient city of Anuradhapura.
Indian sources also said Mohammad Muhsin Nilam- aka Abu Shurayh- who was closely linked to the NJT died in an air strike on Raqqa in northeast Syria in 2015.
Islamic State gave the noms de guerre of each of the seven bombers and claimed to identify where each of them had detonated. All their faces were covered by scarves except Hashmi's. Sri Lankan intelligence sources said it was still unclear if Hashmi had been among the attackers, or had masterminded the carnage and then gone on the run.
The family of a businessman who owned a copper factory in Colombo told the Daily Mail he had been one of the suicide bombers. Inshaf Ahamad blew himself up at one of the three hotels attacked, after telling his wife he was going on a business trip to africa and she had to “be strong”. His brother is also thought to have been an attacker.
Nine of his factory staff were arrested as detectives tried to determine if they were involved, or if the factory was used to assemble the bombs, possibly using the powerful homemade explosive known as 'Mother of Satan'.
Ahamad's brother is also thought to have been one of the attackers. The brothers were believed to be from a wealthy Colombo family.
Hasmi himself was known among the Muslim community as a divisive figure who was said to have dropped out of his seminary in India either because of ideological differences or over money worries. He is believed to have clashed with fellow clerics and encouraged his followers to attack rival mosques.
Hilmy Ahamed, the vice-president of the Muslim Council of Sri Lanka, told The Telegraph he had been trying to warn officials about Hashmi’s extremism for three years.
"We were very concerned that this guy was preaching hate on social media and uploading a lot of videos,” he said.
His videos saw him preach hate against other religions, in one case standing in front of a backdrop of the burning World Trade Centre.
Mr Ahamed said Hashmi continued to shuttle between India and Sri Lanka, travelling by fishing boat to avoid detection.
Hashmi's group began as an offshoot of the Sri Lanka Thawheed Jamaath, which has repeatedly fractured due to internal disputes.
The group could not have carried out the attack without external help, added Mr Ahamed.
British intelligence sources were deeply sceptical of Sri Lankan government claims the bombings were carried out in retaliation for the Christchurch mosque attacks in New Zealand because that would have given them little time to plan the attack.
A second source suggested that working detonators - key to carrying out the suicide attacks - were likely acquired in Sri Lanka as a leftover from the civil war - rather than smuggled in by Isil terrorists.
As Sri Lanka remained tense and faced another security curfew overnight, police at one point warned that two vehicles packed with explosives were thought to be driving around the capital.