The death toll in the Sri Lankan attacks has risen to 310, police sources have revealed.
Police have arrested 40 suspects in the Sri Lanka bombings, including the driver of a van allegedly used by the suicide bombers and the owner of a house where some of them lived.
Members of three British families were among nearly 300 people killed in the series of terror attacks in Sri Lanka.
Lawyer Ben Nicholson lost his wife Anita, son Alex, 14, and daughter Annabel, 11, when one of seven suicide bombers struck as they ate breakfast at the Shangri-La hotel in Colombo.
Londoner Matthew Linsey's daughter Amelie, 15, and son Daniel, 19, were killed in the same blast on the final day of their holiday.
GP Sally Bradley and her husband Bill Harrop, a retired firefighter, from Manchester, died in the Cinnamon Grand Hotel bombing.
Sri Lanka's president gave the military a wider berth to detain and arrest suspects on Tuesday - powers that were used during the 26-year civil war but withdrawn when it ended in 2009.
Police spokesman Ruwan Gunasekara said the death toll from Sunday's attacks had risen to 310.
President Maithripala Sirisena declared a day of mourning on Tuesday.
Sri Lankan authorities planned to brief foreign diplomats and receive assistance from the FBI and other foreign intelligence-gathering agencies.
Sri Lanka attacks: Brit siblings escaped first bomb but are killed by second blast
The six near-simultaneous attacks on three churches and three luxury hotels, along with three related blasts later on Sunday, were the South Asian island nation's deadliest violence in a decade.
Sri Lankan police are holding a Syrian national in custody for questioning military sources told Reuters on Tuesday.
"The terrorist investigation division of the police arrested a Syrian national following the attacks for interrogation," a source said.
Two other officials with knowledge of the investigation confirmed the detention.
"He was arrested after interrogation of local suspects," a second source said.
Prime minister Ranil Wickremesinghe said he feared the massacre could unleash instability and he vowed to "vest all necessary powers with the defence forces" to act against those responsible.
In an indication of the tensions, three explosions caused panic but apparently no injuries on Monday as police were defusing bombs inside a van parked near one of the stricken churches.
Dozens of detonators were discovered near Colombo's main bus depot, but officials declined to say whether they were linked to the attacks.
Sri Lanka terror attack warnings were ignored - including tip-off weeks earlier
International intelligence agencies had warned that the little-known group, National Thowfeek Jamaath, was planning attacks, but word apparently did not reach the prime minister's office until after the massacre.
Health minister Rajitha Senaratne said the warnings started on April 4, the defence ministry wrote to the police chief with information that included the group's name and and police wrote on April 11 to the heads of security of the judiciary and diplomatic security division.
Mr Sirisena, who was out of the country on Sunday, had ousted Mr Wickremesinghe in October and dissolved the Cabinet.
The Supreme Court reversed his actions, but the prime minister has not been allowed into meetings of the Security Council since October, leaving him and his government in the dark about the intelligence.
It was not immediately clear what action, if any, was taken after the threats. Authorities said they knew where the group trained and had safe houses, but did not identify any of the suicide bombers, whose bodies were recovered, or the two dozen other suspects taken into custody.
All the bombers were Sri Lankans, but authorities said they strongly suspected foreign links, Mr Senaratne said.
Also unclear was a motive. The history of Buddhist-majority Sri Lanka, a country of 21 million including large Hindu, Muslim and Christian minorities, is rife with ethnic and sectarian conflict.
In the 26-year civil war, the Tamil Tigers, a powerful rebel army known for using suicide bombers, was finally crushed by the government in 2009 but had little history of targeting Christians.
Anti-Muslim bigotry fed by Buddhist nationalists has swept the country recently, but there is no history of Islamic militancy. Its small Christian community has seen only scattered incidents of harassment.
Two of the stricken churches are Catholic and one Protestant. The three hotels and one of the churches, St Anthony's Shrine, are frequented by foreigners.
Tourism minister John Amaratunga said 39 foreigners were killed, although the foreign ministry gave the figure as 31. The reason for the discrepancy was not clear, but some victims were dual nationals.
India and Britain have each confirmed eight dead. The US State Department confirmed that at least four Americans were dead and several seriously wounded.
Others were confirmed to be from Bangladesh, China, France, Japan, the Netherlands, Denmark, Portugal, Saudi Arabia, Spain, Turkey and Australia.
The six near-simultaneous blasts were set off on Sunday morning at St Anthony's and the Cinnamon Grand, Shangri-La and Kingsbury hotels in Colombo, as well as the two churches outside Colombo.
The military confirmed two other related blasts, one near an overpass and another at a guesthouse where two people were killed. A ninth blast, which killed three police officers, was set off by occupants of a safe house trying to evade arrest, authorities said.
