No group has claimed responsibility for the attacks, which officials said were carried out by at least seven suicide bombers on three churches and four hotels. About 500 people were also wounded.
However, the focus of suspicion is falling on Islamist militants with links to foreign groups. U.S. intelligence sources said the attacks bore some of the hallmarks of the Islamic State extremist group.
Tuesday was declared a national day of mourning and the funerals of some of the victims were held as pressure mounted on the government over why effective action had not been taken in response to a warning this month about a possible attack on churches by a little-known domestic Islamist group.
Police said the number of people arrested since Sunday had risen to 40, most of them Sri Lankans. The investigation of those detainees had led to the Syrian, three government and military sources told Reuters.
"He was arrested after the interrogation of local suspects," one of the sources said.
The first six attacks - on three churches and three luxury hotels - came within 20 minutes on Sunday morning during Easter services and as hotels served breakfast.
Two more explosions - at a down-market hotel and a house in a suburb of the capital, Colombo - came in the early afternoon.
Most of the dead and wounded were Sri Lankans, although government officials said 38 foreigners were killed. That included British, U.S., Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals.
FBI TO HELP
The bombs brought a shattering end to a relative calm that had existed in the Buddhist-majority Indian Ocean island since a bitter civil war fought by Tamil separatists ended 10 years ago and raised fears of a return to sectarian violence.
Sri Lanka's 22 million people include minority Christians, Muslims and Hindus. Up to now, Christians had largely managed to keep out of the worst of the island's conflict and communal tensions.
The government imposed an emergency law at midnight on Monday, giving police extensive powers to detain and interrogate suspects without court orders.
An overnight curfew has also been imposed since Sunday.
U.S. President Donald Trump called Prime Minister Ranil Wickremesinghe on Monday to pledge U.S. support in bringing the perpetrators to justice.
The Washington Post quoted an unidentified law enforcement official as saying Federal Bureau of Investigation (FBI) agents were being sent to Sri Lanka to assist in the investigation.
The FBI has also offered laboratory expertise to test evidence and analysts were scouring databases for information that might shed light on the attacks, the Post said. Counter-terrorism officials from Britain were also due to arrive on Tuesday, a Western diplomat in Colombo said.
U.S. intelligence sources said the attacks carried some of the hallmarks of the Islamic State militant group, although they were cautious because it had not claimed responsibility.
Islamic State is usually quick to claim responsibility for, or links to, attacks against foreign targets or religious groups whether they were involved or not.
Footage on CNN showed what it said was one of the bombers wearing a heavy backpack. The man patted a child on the head before entering the Gothic-style St. Sebastian church in Katuwapitiya, north of Colombo. Dozens were killed there.
INTERNAL FEUD
The attacks have also underlined concern over fractures in Sri Lanka's government, and whether the discord prevented action that might have stopped them.
The government got a tip-off from India this month about a possible attack on churches by a little-known domestic Islamist group, the National Thawheed Jama'ut group.
It was not immediately clear what action, if any, was taken in response. A government minister said on Monday Wickremesinghe had not been informed about the warning and had been shut out of top security meetings because of a feud with President Maithripala Sirisena.
Sirisena fired Wickremesinghe last year only to be forced to reinstate him under pressure from the Supreme Court. Their relationship is reported to be fraught.
The U.S. State Department said in a travel advisory "terrorist groups" were plotting possible attacks in Sri Lanka and targets could include tourist spots, transport hubs, shopping malls, hotels, places of worship and airports.
China's embassy in Sri Lanka warned Chinese nationals against traveling to Sri Lanka in the near term because of "huge security risks."
China is a major investor in Sri Lanka. The embassy said one Chinese national was killed, five wounded and five were missing.
Among the victims were three of the four children of Anders Holch Povlsen, Denmark's richest man.
Eight Britons were also killed, including Anita Nicholson, her 14-year-old son and her 11-year-old daughter. Nicholson's husband survived the attack on the Shangri-La Hotel in Colombo.