Police in Londonderry have arrested a further two men in connection with a bomb in the city on Saturday.
The men, aged 34 and 42, were arrested in the city on Sunday evening. It followed the arrest of two men in their 20s earlier in the day.
The PSNI said the attack may have been carried out by dissident republican group the New IRA.
A pizza delivery van was hijacked by two armed men in Derry at about 18:00 GMT.
The bomb, which went off at 20:09 GMT, was described as a "crude device". The PSNI said the attack outside the courthouse was "unbelievably reckless".
PIC 1: 1923hrs - driver abandons car outside Derry/Londonderry court house. PIC 2: 1940hrs - a group of teenagers spotted wandering past the bomb vehicle. PIC 3: 2009hrs bomb explodes. @BBCNews @BBCNewsNI pic.twitter.com/wbVbnOTnPl— Declan Harvey (@NewsDeclan) January 20, 2019
The police have released CCTV footage of the moment the bomb exploded.
One of the clips, posted on Twitter, shows a group of young people walking past the vehicle shortly before the blast.
Assistant Chief Constable Mark Hamilton said the van, with the bomb inside, was left outside the courthouse on Bishop Street at 19:23 GMT.
Three minutes later, a warning was called into the Samaritans in the West Midlands. It was passed to West Midlands police, who contacted the Police Service of Northern Ireland.
"In the intervening minutes we had already found the car and were starting to evacuate the area," said ACC Hamilton.
"Clearly, it was a very significant attempt to kill people here in this community.
"Thankfully, the local community and the police service acted bravely together and we got everybody away just in time.
"But the bomb detonated just as we were leaving the area.
"The new IRA, like most dissident republican groups in Northern Ireland, are small, largely unrepresentative and determined to drive people back to somewhere they do not want to be."
A cordon remains in place at the scene.
What is the New IRA?
- The new IRA was formed in 2012 after a number of dissident republican organisations said they were unifying under one leadership
- It is believed to be the largest dissident republican organisation
- The group is believed to have been responsible for a number of attacks since its formation, including the murders of prison officers David Black and Adrian Ismay.
Residents, hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club were moved out of the area.
Greg McLaughlin, who lives nearby, said his windows shook with the force of the blast.
"It was very, very loud. I knew right away this was a bomb," he said.
"We knew it was quite close.
"You could see the ball of fire on the street. It sounded to me like a very significant blast. I haven't heard anything like it in Derry for quite a while."
ACC Hamilton said dissident republican groups "always aspire to do bigger things".
He said the device "has not been as effective as they would have wanted for it to be".
"They have not killed anybody and they haven't caused widespread damage, " he said.
Colin Neill, chief executive of Hospitality Ulster, said businesses were determined to trade as normal despite disruption in the area.
He said the bomb "will not deter us from opening today and getting on with the job".
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