Sri Lankan authorities believe the people are ultimately linked to the Islamic State group who claimed responsibility for the attack.
Sri Lankan police are trying to track down 140 people believed to be linked to the Islamic State group, which claimed responsibility for the Easter Sunday suicide bombings of churches and hotels that killed 359 people, President Maithripala Sirisena said Friday.
Muslims in Sri Lanka were urged to pray at home after the State Intelligence Services warned of possible car bomb attacks, amid fears of retaliatory violence.
The United States Embassy in Sri Lanka also urged its citizens to avoid places of worship this weekend after authorities reported there could be more attacks targeting religious centers.
Catholic Archbishop of Colombo Cardinal Malcolm Ranjith told reporters he had seen a leaked security document warning of further attacks on churches and there would be no masses this Sunday anywhere on the island.
The streets of Colombo were deserted Friday evening, with many people leaving offices early amid tight security after several suicide bombings took place Easter Sunday on three churches and four hotels that also wounded about 500 people.
Nearly 10,000 soldiers were deployed across the Indian Ocean island state to carry out searches and provide security for religious centers, the military said on Friday.
The All Ceylon Jamiyathul Ullama, Sri Lanka's main Islamic religious body, urged Muslims to conduct prayers at home in case "there is a need to protect family and properties".
Police have detained at least 76 people, including foreigners from Syria and Egypt, in their investigations so far.
Islamic State provided no evidence to back its claim that it was behind the attacks. If true, it would be one of the worst attacks carried out by the group outside Iraq and Syria.
The extremist group released a video on Tuesday showing eight men, all but one with their faces covered, standing under a black Islamic State flag and declaring their loyalty to its leader, Abu Bakr Al-Baghdadi.
The Sri Lankan government said nine well-educated suicide bombers carried out the attacks, eight of whom had been identified. One was a woman.
The president told reporters on Friday some Sri Lankan youths had been involved with Islamic State since 2013. He said information uncovered so far suggested there were 140 people in Sri Lanka involved in Islamic State activities. "Police are looking to arrest them," Sirisena said.
Authorities have so far focused their investigations on international links to two domestic Islamist groups - National Thawheed Jama'ut and Jammiyathul Millathu Ibrahim - they believe carried out the attacks.
Government officials have acknowledged a major lapse in not widely sharing an intelligence warning from India before the attacks.
President Sirisena asked for the resignation of a top defense aid and police chief for not sharing information with him about the impending attacks. "The police chief said he will resign now," Sirisena has announced.
He blamed Prime Minister Ranil Wickremesinghe's government for weakening the intelligence system by focusing on the prosecution of military officers over alleged war crimes during a decade-long civil war with Tamil separatists that ended in 2009.
That year marked the end of a 26-year war between Indian forces and the Liberation Tigers of Tamil Elam (LTTE) in Sri Lanka that sought a Tamil homeland mainly in northern and eastern parts of the island after the Tamil, who are mainly Hindi and Christian, had experienced decades of religious and cultural oppression.
The Easter Sunday bombings shattered the relative calm that had existed in Buddhist-majority Sri Lanka since the civil war ended.
Opposing factions aligned to either Wickremesinghe or Sirisena have often refused to communicate with each other and blame any setbacks on their opponents, government sources say.
Cardinal Ranjith said that the church had been kept in the dark about intelligence warning of attacks.
“We didn’t know anything. It came as a thunderbolt for us,” he said.
Death Toll in Sri Lanka Rises to 359
Sri Lanka's 22 million people include minority Christians, Muslims and Hindus, where Buddists are the majority. Until now, Christians had largely managed to avoid the worst of the island's conflict and communal tensions.
Most of the victims were Sri Lankans, although authorities said at least 38 foreigners were also killed, many of them tourists at top-end hotels when the bombers struck. Britain warned its nationals on Thursday to avoid Sri Lanka unless it was absolutely necessary.
Fears of retaliatory sectarian violence have already caused Muslim communities to flee their homes amid bomb scares, lockdowns and security sweeps.
But at the Kollupitiya Jumma Masjid mosque in Colombo, hundreds attended a service they say was focused on a call for people of all religions to help return peace to Sri Lanka.
"It's a very sad situation," said 28-year-old sales worker Raees Ulhaq outside the mosque as police dogs sniffed the belongings of worshipers inside.
"We work with Christians, Buddhists, Hindus. It has been a threat for all of us because of what these few people have done to this beautiful country."