One of the Sri Lanka bombers is thought to have studied in the U.K. and Australia before carrying out the Easter Sunday attack, which killed at least 359 people.
The Islamic State group has claimed responsibility for the bombings, which were carried out by a group of suicide bombers with international connections, said Ruwan Wijewardene, Sri Lanka's state minister of defence.
He told reporters at a briefing on Wednesday: "We believe that one of the suicide bombers studied in the U.K. and later did his postgraduate [studies] in Australia before coming back and settling in Sri Lanka."
He added: "This group of suicide bombers, most of them are well-educated and come from middle or upper-middle class, so they are financially quite independent and their families are quite stable financially, that is a worrying factor in this."
Some of the seven bombers involved in the Easter Sunday attacks in Sri Lanka had studied law degrees in "various other countries," the country's state defence minister said. All were Sri Lankan.
Meanwhile, Sri Lankan President Maithripala Sirisena has promised "stern action" and a "complete restructuring" of security forces after it emerged that officials failed to pass on intelligence warnings about the attack.
A further 18 people have been arrested following what was the worst violence seen in Sri Lanka seen since its civil war ended a decade ago. In total, 58 people have been held.
On Wednesday, officials said the death toll had risen to 359.
Specialist officers from the Metropolitan Police's Counter Terrorism Command have been sent to the country to support the bereaved, while the FBI is assisting in the investigation.
Armed suspects may be at large
Sri Lankan police carried out a controlled explosion of a suspicious motor scooter parked near the popular Savoy cinema in the capital Colombo.
There were no explosives in the vehicle, police said.
The government has said the attacks were carried out by Islamic fundamentalists in apparent retaliation for the New Zealand mosque massacre last month, in which 50 people were killed. However, they did not disclose any evidence.
New Zealand Prime Minister Jacinda Ardern said she had not received any official advice from Sri Lanka or seen any intelligence to corroborate the claims.
Sirisena had warned on Tuesday that several suspects armed with explosives were still at large.
By now it has been established that the intelligence units were aware of this attack and a group of responsible people were informed about the impending attack.Ruwan Wijewardene, Sri Lanka's state defence minister
The Islamic State group has lost all the territory it once held in Iraq and Syria and has made a series of unsupported claims of responsibility around the world.
Sri Lankan authorities have blamed a local extremist group, National Towheed Jamaar (NTJ), whose leader, alternately known as Mohammed Zahran or Zahran Hashmi, became known to Muslim leaders three years ago for his incendiary speeches online.
In an address to parliament, Wijewardene said "weakness" within Sri Lanka's security apparatus led to the failure to prevent the nine bombings.
"By now it has been established that the intelligence units were aware of this attack and a group of responsible people were informed about the impending attack," Wijewardene said.
"However, this information has been circulated among only a few officials."