Sri Lanka bombings: Who is responsible? And did the government know an attack was coming?

At least 290 people, including four Americans, were killed in a string of suicide bombings carried out by a domestic militant group at churches and luxury hotels on Easter Sunday in Sri Lanka, government officials said Monday.

All seven suicide bombers were Sri Lankan citizens and from the radical Muslim group National Thowfeek Jamaath, but officials suspect foreign links, Health Minister Rajitha Senaratne told reporters.

The attack, which also injured more than 500 people, was the deadliest violence in Sri Lanka, the South Asian island nation, since its civil war ended a decade ago, police spokesperson Ruwan Gunasekara said.

Here's what we know now:

What happened?

Six nearly simultaneous explosions hit three hotels and three churches as worshippers gathered for Easter services in the cities of Colombo, Negombo and Batticaloa.

Hours later, there were two more blasts, one at a guesthouse that killed two people and another near an overpass in the area of Dematagoda on the outskirts of the capital of Colombo, according to a Sri Lankan military spokesman, Brig. Sumith Atapattu.

Authorities say three police officers were also killed when suspects at a safe house detonated explosives.

Ariyananda Welianga, a government forensic crime investigator, said all the attacks were carried out by a single bomber, with two at Colombo’s Shangri-La Hotel.

“People were being dragged out,” said Bhanuka Harischandra of Colombo, a 24-year-old founder of a tech marketing company who was going to the city’s Shangri-La Hotel for a meeting when it was bombed. “People didn’t know what was going on. It was panic mode.”

He added: “There was blood everywhere.”

Police found more bombs Monday, some exploded

A van parked outside one of the churches that was bombed exploded Monday, but no one was injured.

Police found three bombs on the van that had been parked outside since Sunday. They were trying to defuse the bombs when the van was detonated.

Officials also say they've found 87 bomb detonators in Colombo. Twelve were found at the city's main bus depot and 75 more at a garbage dump in the area.

President Maithripala Sirisena said late Monday that he had given the military war-time powers to arrest suspects.

Who is dead?

Most of the 290 people killed in the attacks were Sri Lankans.

At least 30 foreign tourists were killed and 28 were wounded, government officials said, with the fatality figures ranging from 31 to 39. The U.S. State Department said at least four Americans were among the dead and several more were seriously injured.

Dieter Kowalski, 40, a Wisconsin native living in Colorado, was among the Americans killed, his employer, education company Pearson, confirmed Monday.

And officials at Sidwell Friends, the Washington-area private school where former President Obama's daughter Sasha is a senior, confirmed the death of one of its students in an email to parents.

The Washington Post reported the fifth-grade boy, Kieran Shafritz de Zoysa, had been on leave for the last year in Sri Lanka.

Victims from the United Kingdom, India, Denmark, Australia, China, Japan, Spain and Portugal were also confirmed.

Who is responsible?

Officials say the local militant group National Thowfeek Jamaath carried out the attacks.

At least 24 people were in custody for questioning Monday and there were seven confirmed suicide bombers, authorities said. However, no group has claimed responsibility.

Officials also believe the group was working as part of a larger international network.

Did Sri Lanka know an attack was coming?

Sri Lankan government officials have said there was some indication an attack could occur.

International intelligence agencies began warning the country's officials on April 4, and on April 9 the defense ministry included the group's name in a warning to the police chief, Senaratne said. An April 11 report also warned of an attack, Senaratne said.

"We placed our hands on our heads when we came to know that these deaths could have been avoided," said Cardinal Malcolm Ranjith, the archbishop of Colombo.

How is the US responding?

U.S. Secretary of State Mike Pompeo condemned the attacks in a statement.

"Attacks on innocent people gathering in a place of worship or enjoying a holiday meal are affronts to the universal values and freedoms that we hold dear, and demonstrate yet again the brutal nature of radical terrorists whose sole aim is to threaten peace and security," he said.

The State Department also issued a travel advisory Sunday to "exercise increased caution in Sri Lanka due to terrorism."

President Donald Trump tweeted his condolences to the victims Sunday morning, initially stating that the explosions killed "at least 138 million people." The inaccurate tweet stayed up for about 20 minutes before it was deleted, and a new statement was issued with the same pledge to help and the accurate number of fatalities.

Where is Sri Lanka?

The island nation of about 21 million people is just off the southern tip of India.

It has been relatively peaceful since a 16-year civil war ended in 2009, though its various factions continued to jostle for power.

The majority are Sinhalese, mostly Buddhist. The minority Tamil are Hindu, Muslim and Christian. Christians, targeted in Sunday's attacks, have a lower profile than some of the other factions.

Contributing: Kirk Bado and Chris Woodyard, USA TODAY; The Associated Press

This article originally appeared on USA TODAY: Sri Lanka bombings: Who is responsible? And did the government know an attack was coming?