On Easter Sunday, the country was shaken by a series of coordinated attacks, which killed at least 290 people and left another 500 injured.
Politicians emerged from an emergency meeting pointing fingers at a little-known local Muslim group, National Thowfeek Jamaath.
The attacks on Easter Sunday, which Sri Lankan authorities believe were carried out by a local Islamist group, National Thowheeth Jama'ath (NTJ), were "one of the deadliest terrorist events since the Sept 11, 2001, attacks on the United States", White House spokesman Hogan Gidley said.
The detonators were discovered on Monday at a private bus stand located in the Pettah neighborhood of Colombo, police said.
Police found 12 detonators scattered at Colombo's main bus depot on Monday and hours later found another 75 detonators at a garbage dump in the same area.
Funerals began on Monday, as more of the victims were identified.
The Sri Lankan government has also blocked all social media in a bid to "ease tensions".
The U.S. said "several" Americans were among the dead.
Here are some key things to know. The capital, Colombo, took the worst hit.
Who is responsible for the attacks?
No group has claimed responsibility for the attacks yet. Not much is known about NTJ, which only came to attention previous year when it was blamed for defacing some Buddhist statues.
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National Thowheed Jamath is a relatively unknown group that has not been linked to any previous major attacks. The government imposed a weeklong ban in March 2018 because of concerns that WhatsApp and other platforms were being used to fan anti-Muslim violence in the country's central region. "We are now investigating worldwide support for them and their other links - how they produced the suicide bombers and bombs like this".
Was there any warning about the attacks?
Prime Minister Ranil Wickremesinghe, whom the president attempted to have fired from his position in October 2018, urged a review into what was done with the intelligence.
Mr Sirisena sacked Mr Wickremesinghe and his cabinet in October and tried to install another prime minister, prompting a full-blown constitutional crisis.
For many, Sunday's attacks evoked the memory of the Sri Lanka's almost three-decade civil war between ethnic Tamil separatists and government forces.
Sri Lanka only recently recovered from a brutal 26-year civil war between the predominately Buddhist Sinhalese majority and mostly Hindu Tamil minority. The Sri Lanka Red Cross Society is taking donations here.
Many came to a section of Tompkinsville that became known as "Little Sri Lanka".
The 1.5 million Christians in the mostly Buddhist country make up about 7 percent of the population. Tamils are mostly Hindu, and Hindus represent almost 13 percent of the population.
Last year, there were suicide attacks on churches in the Indonesian city of Surabaya and in the town of Jolo in the Philippines this year.
Speaking at one of the bomb sites, Sri Lanka's Defence Secretary Hemasiri Fernando disputed any suggestion there had been an intelligence failing.