CSI officers examine the scene of the bomb last in hijacked vehicle outside the Courthouse in Derry. Pic by North West Newspix
Bishop Street Courthouse will remain closed following the blast on Saturday.
The PSNI said the attack may have been carried out by the New IRA, a dissident republican group, and have arrested four men – two aged in their 20s, one aged 32 and another aged 42.
They were detained under Section 41 of the Terrorism Act and have been taken to the serious crime custody suite at Musgrave PSNI station for questioning.
They said a pizza delivery man had a gun held to his head when his van was hijacked for the bombing.
The police said the area will remain closed from Bishop’s Gate down to the Bishop’s Gate Hotel.
At the courthouse, jury trials due to be held have been put off until Tuesday.
Cases listed to be heard in the magistrate’s courts will now be held in Strabane, with some other hearings switching to Coleraine.
Bishop Street car park remains closed with no access to vehicles or pedestrians.
In a statement on Sunday night, the PSNI said: “We are aware that a number of people are seeking the return of their vehicles – unfortunately this cannot be facilitated due to the continuing police operation.
“We would like to thank the community for their patience and support.”
The police released CCTV footage of the moment the bomb exploded in a car that had previously been hijacked in Quarry Street earlier on Saturday evening.
Another CCTV clip, posted on Twitter, showed a group of young people walking past the vehicle shortly before the bomb detonated.
PSNI Supt Gordon McCalmont told BBC Radio Foyle that the police were “trying to get the city back to normal and show that this attack has had little or no long-term impact”.
He said the PSNI “reflected long and hard on whether or not” to release the CCTV footage of the explosion and added it was “lucky we are not talking about loss of life”.
Supt McCalmont also said the pizza delivery van driver “had to go through the drama of having a firearm put to his head”.
“My thoughts are with that man today, He was threatened and intimidated. It would be fair to say he was asked not to raise the alarm.”
He added: “We will obviously review how we deal with these situations but these groupings obviously want us to respond.
“We will be very balanced. This threat has always been in this city.
“My sense is that this is not because of Brexit.”
Two further men arrested in searches in Derry over bomb attack in Bishop Street on Saturday night
