Islamic State has claimed responsibility for the terrorist blasts in Sri Lanka, as investigations into Sunday’s coordinated attacks that killed 321 people intensify, Bloomberg reports.

The group’s Amaq News sent a tweet citing a "security source" as saying the attacks were the work of "fighters of the Islamic State."

Interpol has joined the investigation to help identify potential international connections, with attention also focused on a second extremist group known as Jamaat-ul-Mujahideen.

In a special session of the parliament today, State Minister of Defence Ruwan Wijewardene said investigators were probing links between the local jihadist group National Thowheed Jamath and Jamaat-ul-Mujahideen. He noted the Easter Sunday bombings could have been retaliation for the terrorist attacks on two New Zealand mosques last month.

Prime Minister Ranil Wickremesinghe vowed that Sri Lanka would not allow the attacks to lead to another war, referring to the three-decade civil war that ended in 2009.

"The intelligence agencies have reported that there were international organizations behind these acts of local terrorists," President Maithripala Sirisena said in a statement. "Hence, it has been decided to seek international assistance for investigations."

The government said other nations had shared intelligence ahead of the blasts. Seven suicide bombers carried out the Easter Sunday assault on churches and luxury hotels, targeting Christians and foreign tourists, it said.

Over 40 suspects were in custody, national police spokesman Ruwan Gunasekera said Tuesday.