Sri Lankan authorities have said one of nine suicide bombers who carried out the Easter Sunday terror attacks which left 359 people dead was educated in the UK.
Police have now arrested 60 people in connection with the bombings.
Other members of the terror cell had also studied abroad, said minister Ruwan Wijewardene.
Another of the nine bombers was a woman and is believed to be the wife of another bomber, Mr Wijewardene said.
“We are talking about a group that was well educated, mostly middle class and stable financially,” he said.
UK intelligence officers were assisting Sri Lanka with its investigations, the minister said, adding that the British government has been “very helpful”.
The US is also helping Sri Lankan authorities investigate the bombings, and will include the FBI.
The US ambassador to Sri Lanka also said given the coordination and scale of the attacks, the involvement of groups such as Isis need to be investigated.
Despite the US having no prior knowledge of the attacks, Washington’s ambassador said America now believes there is ongoing terrorism plotting in the country.
“We had no prior knowledge of these attacks,” ambassador Alaina Teplitz told reporters in Colombo.
“We believe there are ongoing terrorist plots. Terrorists can strike without warning. Typical venues are large gatherings, public spaces.”
Speaking to parliament on Wednesday, Sarath Fonseka, former army chief and minister of regional development, said he believed the attacks had been planned for at least 7-8 years.
The leader of Sri Lanka’s parliament, accused the country’s intelligence agencies of purposefully withholding prior information about the attacks for political reasons.
“Some top intelligence officials hid the intelligence information purposefully. Information was there, but the top brass security officials did not take appropriate actions,” Lakshman Kiriella, who is also minister of public enterprise, told parliament.
He said information on possible suicide attacks on churches, hotels and politicians were received from Indian intelligence on 4 April and a security council meeting was chaired by President Maithripala Sirisena on 7 April, but the information was not shared more widely.
“Somebody is controlling these top intelligence officials,” the minister said. “The Security Council is doing politics. We need to investigate into this.”
In the country’s capital, Colombo, a small controlled explosion was carried out on Wednesday morning on a “suspicious vehicle” parked outside a cinema.
Agencies contributed to this report.