Police in Derry in Northern Ireland 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.
They 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 (7am NZT).
The bomb, which went off at 20:09 GMT (9am NZT), was described as a "crude device". The Police Service of Northern Ireland (PSNI) said the attack outside the courthouse was "unbelievably reckless".
The police have released CCTV footage of the moment the bomb exploded.
Police release CCTV footage of explosion outside courthouse on Bishops Street in Derry/Londonderry last night. pic.twitter.com/tqzqBdCZnv— PSNI DC&S District (@PSNIDCSDistrict) January 20, 2019
One of the clips, posted on Twitter, showed 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.
Police release CCTV footage of a group of young people walking past car outside courthouse on Bishops Street in Derry/Londonderry last night. pic.twitter.com/DL0hnfX8Xj— PSNI DC&S District (@PSNIDCSDistrict) January 20, 2019
Three minutes later, a warning was called into the Samaritans in the West Midlands. It was passed to West Midlands police, who contacted the PSNI.
"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.
Police release CCTV footage of driver running away from car outside courthouse on Bishops Street in Derry/Londonderry last night. pic.twitter.com/0nXw7qzDt9— PSNI DC&S District (@PSNIDCSDistrict) January 20, 2019
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".