Catholics in Sri Lanka held their first Sunday mass under tight security on Sunday, three weeks after 258 people were killed in a slew of ISIS-claimed suicide bomb attacks at churches and luxury hotels on Easter Sunday. Thousands of people in Colombo attended the weekly service at St Theresa's Church, protected by soldiers armed with assault rifles, while members of the congregation were searched for explosives. A special service was held for those killed and injured on the April 21 attack at St Lucia's Cathedral on Saturday, attended by the relatives of victims and survivors of the bombings. The government blamed local Islamist militants, who had pledged allegiance to ISIS, for the bombings on the Catholic holiday in which a series of bombs targeted ongoing services and five-star hotels. The Sunday mass in Sri Lanka's capital is the first publicly celebrated mass since the bombings, the last two have been held in private but broadcast on national television. Regular services were cancelled across all churches soon after the deadly suicide attacks, but the archbishop of Colombo Cardinal Malcolm Ranjith announced on Thursday that mass would be held in his diocese from Sunday. Catholic private schools, which were closed for the Easter holidays, will reopen on Tuesday, church officials said. State schools resumed classes last week, guarded by police and security forces. But despite the safety measures, attendance remains low. The government blamed National Thowheeth Jama'ath (NTJ) for the bombings, banning the little-known group in a raft of emergency laws passed to deal with the attacks. The bombers pledged allegiance to ISIS's leader Abu Bakr Al Baghdadi before the attacks, and ISIS claimed responsibility. Sri Lanka's police said they have either killed or arrested all of the militants responsible for the attacks, but warned against the threat of global terrorism.