ISIS has claimed the Sri Lankan bombers operated as part of its franchise.

One month after the Sri Lanka suicide attacks that killed more than 250 people, investigators have told AFP the bombers used "Mother of Satan" explosives favoured by the ISIS group that are a new sign of foreign involvement.

Detectives said the back-pack bombs used in the April 21 attacks on three churches and three hotels were manufactured by local jihadists with ISIS expertise.

They named the explosive as triacetone triperoxide, or TATP, an unstable but easily made mixture favoured by ISIS militants who call it "Mother of Satan".

It was also used in the 2015 attacks in Paris, by a suicide bomber who hit the Manchester Arena in England in 2017 and attacks on churches in Indonesia one year ago.

ISIS has claimed the Sri Lankan bombers operated as part of its franchise. But Sri Lankan and international investigators are anxious to know just how much outside help went into the attacks that left 258 dead and 500 injured.

"The group had easy access to chemicals and fertiliser to get the raw materials to make TATP," an official involved in the investigation told AFP.

Sri Lankan detectives say the National Thowheeth Jama'ath (NTJ), local militants blamed for the attacks, must have had foreign help to assemble the bombs.

Multinational militants

"They would have had a face-to-face meeting to transfer this technology. This is not something you can do by watching a YouTube video," said the official, speaking on condition of anonymity.

Investigators had initially believed that C4 explosives -- a favoured weapon of Tamil Tiger rebels -- were used, but forensic tests found TATP which causes more burning than C4.

Police have also confirmed that 100 kilograms (220 pounds) of explosives found in January in the island's northwest was TATP.

They are checking the travel records of the suicide bombers as well as foreign suspects to see when and where bomb-making lessons could have been staged.

"It looks like they used a cocktail of TATP and gelignite and some chemicals in the Easter attacks. They were short of the 100 kilos of raw TATP that were seized in January," said the investigator.

Sri Lankan security forces have staged a series of raids since the bombings. Police spokesman Ruwan Gunasekera said Sunday that 89 suspects are in custody.

Army chief Mahesh Senanayake said last week that at least two suspects have been arrested in Qatar and Saudi Arabia, underscoring the international link.

On April 26, six militants, three widows of the suicide bombers and six of their children were killed at an NTJ safe house near the eastern coastal town of Kalmunai.

Police found large quantities of chemicals and fertilizer there that was probably meant to make bombs, authorities said.

The government has admitted that Indian warnings of the looming attacks in early April were ignored.

But President Maithripala Sirisena has said eight countries are helping the investigation. A US Federal Bureau of Investigation team is in Sri Lanka and Britain, Australia and India have provided forensic and technical support.

China offered a fleet of vehicles to bolster the mobility of the security forces tracking down militants.

'Contact with Islamists'

The Sri Lankan who led the attacks, Zahran Hashim, was known to have travelled to India in the months before he became one of the suicide bombers.

Moderate Muslims had warned authorities about the radical cleric who first set off alarm bells in 2017 when he threatened non-Muslims.

He was one of two bombers who killed dozens of victims at Colombo's Shangri-La hotel on April 21.

Army chief Senanayake said Hashim had travelled to Tamil Nadu state in southern India and been in contact with Islamists there.

Hashim, one of seven bombers who staged the attacks, also appeared in an ISIS group video that claimed responsibility for the attacks.

Another bomber who was meant to have hit a fourth hotel, has been named as Abdul Latheef Jameel who studied aviation engineering in Britain and Australia.

Authorities in the two countries are investigating whether he was radicalised whilst abroad.

Jameel blew himself up when confronted at a hideout after the attacks.