The latest on Sri Lanka's bombing investigationBy Caitlin Hu, Euan McKirdy and Tara John, CNN
We are concluding our coverage for the day. Read our full report on today's events here.
The UK Foreign Office is warning against "all but essential travel to Sri Lanka," according to its latest travel advice to the South Asian country.
The change is "in light of the ongoing security operation and heightened risk of terrorism in Sri Lanka following the attacks on Easter Sunday," it wrote in a statement on Thursday.
UK Foreign Secretary Jeremy Hunt said:
We will keep this decision under close and constant review. My first priority will always be the security of British citizens living and travelling abroad.
We all hope the situation will return to normal very soon, and that the Sri Lankan tourism industry is able to get back on its feet following the terrorist attacks. We will do all we can to help the Sri Lankan authorities.
The Sri Lankan Prime Minister, Ranil Wickremesinghe, has warned of the potential for more attacks in the country. In an interview with CNN's Ivan Watson, he said authorities were targeting "sleepers" -- terrorists who could activate to initiate another round of attacks.
"Police and security forces are rounding up those involved, but they're also rounding up the sleepers, those used on second and third rounds (of attacks)," he said.
"The danger has come down drastically, (but) we do have to pick up some more sleepers, which we will do in the next few days."
He said that security services were acting out of an abundance of caution, and that should even one militant slip through the net, the damage could be widespread. "It is a precaution that we are taking, we want to be sure we have everyone in," he said in the interview.
"They are worried that one or two could get into a church -- (even) one person can do a lot of damage."
The Archdiocese of Colombo has suspended all Catholic church services until April 29 because of security concerns,.
An island-wide curfew will go into effect from 10 p.m. local time (12:30 p.m. ET) on Thursday until 4 a.m. Friday morning, a police spokesperson said.
There has been a nightly curfew in Sri Lanka since Sunday's attacks.
More than 70 suspects have been arrested across the country since Sunday, Sri Lankan police spokesman, Ruwan Gunasekera, told CNN.
He said the suspects have been arrested on suspicion of terrorism, aiding and abetting and conspiracy to commit terrorism.
He also says the four most serious cases are in the custody of the Terrorism Investigation Department (TID). A further 33 suspects are in custody of the Criminal Investigation Department (CID). All remaining suspects are being held by local police.
The majority of the suspects were arrested in Colombo, four of which are women. All of the suspects are from Sri Lanka and most are family members and friends of the suicide bombers.
Police forces from at least six countries and Interpol are helping Sri Lanka with the investigation into Sunday's bombings, Sri Lankan national police spokesman Ruwan Gunasekera told CNN.
The forces involved are:
- Scotland Yard, UK
- FBI, USA
- New Zealand Police
- Australian Federal Police
- Danish Police
- Dutch Police
One of the Sri Lanka suicide bombers was previously arrested by police and then released, a senior Sri Lankan government spokesman told CNN Thursday.
Ilham Ahmed Ibrahim -- one of two sons of a spice tycoon who blew themselves up in Sunday's attacks -- detonated a device at the Cinnamon Grand hotel in Colombo, Sudarshana Gunawardana said.
"It was the suicide bomber of the Cinnamon Grand bomb attack who was released earlier," Gunawardana said.
Ilham Ahmed Ibrahim and his brother Imsath Ahmed Ibrahim were previously identified as two of the suicide bombers in Sunday's attacks, which left at least 359 people dead across the country.
Police have confirmed to CNN that they are holding the brothers' father, Mohamed Yusuf Ibrahim, a wealthy spice trader, on suspicion of aiding and abetting his sons.
Police spokesman Ruwan Gunasekera said the elder Ibrahim was is in custody, as are all other members of the Ibrahim family police are aware of in the country.
CNN has not been able to reach Mohamed Yusuf Ibrahim or other members of his family for comment.
Israel's Counter-Terrorism Bureau issued a travel warning for Sri Lanka on Thursday, raising the threat level to indicate a "high concrete threat," advising travelers to leave the country and avoid visiting Sri Lanka in the near future.
The elevated alert level comes after consultations with security and foreign ministry officials, according to a statement from the Counter-Terrorism Bureau.
Level 2 is the second-highest threat level. A threat level of 1 indicates a "very high concrete threat" in which travelers are urged to leave the country immediately.
Some of the suspected attackers responsible for the Easter Sunday bombings in Sri Lanka were being monitored by the country’s intelligence services, Sri Lanka’s Prime Minister Ranil Wickremesinghe told CNN's Ivan Watson in an interview Thursday.
There was not "sufficient" evidence to put the suspected attackers in custody prior to the attacks, Wickremesinghe said.
He reiterated that the attackers were middle- and upper-middle class and had been educated abroad. He added that the profile of the suspected bombers was "surprising."