Sri Lankan police have arrested 13 men in connection with bomb blasts on churches and hotels that killed more than 207 people, officials said on Monday.
Authorities have not made public details on those held after Sunday's attacks. But a police source told AFP the 13 were detained at two locations in and around Colombo.
The source said the 13 men are from the same radical group.
At least two of the eight attacks were carried out by suicide bombers, according to police and other sources, and three police were killed when another suicide bomber detonated explosives during a raid on a house where suspects were.
At least 207 people were killed, among them dozens of foreigners.
An improvised bomb discovered at the main airport in Colombo was defused late Sunday, police said.
A nationwide curfew imposed on Sunday shortly after the blasts was lifted early Monday, with AFP journalists reporting a steady stream of people and tuk tuks on the streets of Negombo.
There was still a heavy security presence at the city's St Sebastien's Church, the scene of one of the devastating blasts.
There was no immediate claim of responsibility, but police said Monday 13 people had been arrested. The government earlier said investigators would to look into whether the attackers had "overseas links".
The powerful blasts - six in quick succession and then two more hours later - wounded around 450 people.
At least two of the explosions involved suicide bombers, including one who lined up at a hotel breakfast buffet before unleashing carnage.
The government said the dead included three Indians, three Britons, two from Turkey and one Portuguese national. Two people holding both British and US passports were also among the fatalities.
"Additionally, while nine foreign nationals are reported missing, there are 25 unidentified bodies believed to be of foreigners," the foreign ministry said.
Japan's foreign ministry said one of its nationals was among the dead.
The churches targeted included the historic St Anthony's Shrine in Colombo, where the blast blew out much of the roof.
Bodies lay on the floor of the church, covered in patterned scarves and white sheets, some of them stained with blood.
Shattered roof tiles and shards of glass littered the floor, along with chunks of plaster blasted from the walls by the explosion.
Documents seen by AFP show that Sri Lanka's police chief Pujuth Jayasundara issued an intelligence alert to top officers 10 days ago, warning that suicide bombers planned to hit "prominent churches".
"A foreign intelligence agency has reported that the NTJ (National Thowheeth Jama'ath) is planning to carry out suicide attacks targeting prominent churches as well as the Indian high commission in Colombo," the alert said.
The NTJ is a radical Muslim group in Sri Lanka that was linked last year to the vandalisation of Buddhist statues.
Prime Minister Ranil Wickremesinghe acknowledged that "information was there" about possible attacks and that an investigation would look into "why adequate precautions were not taken".
Sri Lanka's minister of economic reforms Harsha de Silva described "horrible scenes" at St Anthony's church.
"I saw many body parts strewn all over," he tweeted.
Witness N. A. Sumanapala was near the church when the blast happened.
"I ran inside to help. The priest came out and he was covered in blood," he told AFP. "It was a river of blood."
A second blast hit St Sebastian's Church during Easter Mass.
Gabriel, who declined to give his family name, said his brother was injured in the explosion, adding: "We don't want the country to go back to that dark past where we had to live in fear of suicide blasts all the time."
Soon after the first two church blasts, police confirmed that the Zion church in the east coast town of Batticaloa had been hit, along with three high-end hotels in the capital - the Cinnamon Grand, the Shangri-La and the Kingsbury.
A manager at the Cinnamon Grand, near the prime minister's official residence in Colombo, said a suicide bomber blew himself up at the hotel's restaurant.
"He came up to the top of the queue and set off the blast," the manager said.
Later in the afternoon, two people died in a strike at a hotel in the south of Colombo, and a suicide bomber killed three police officers as they raided a house in a northern suburb of the city.
Wickremesinghe urged people to "hold our unity as Sri Lankans" and pledged to "wipe out this menace once and for all".
The archbishop of Colombo, Malcolm Ranjith, described the attackers as "animals" and called on authorities to "punish them mercilessly".
The attacks drew condemnation from around the world, including from US President Donald Trump and the pope.
Embassies in the capital warned citizens to stay inside, while there were chaotic scenes at Colombo airport as travellers formed huge lines at the only taxi counter that was open.