By Aoife Moore and David Young
Four men arrested on suspicion of a car bombing in Londonderry have been released.
The men, aged 42, 34 and two aged 21, were arrested on Sunday by detectives investigating the explosion in Bishop Street, Derry, on Saturday. They have been released unconditionally.
A 50-year-old man arrested on Monday remains in police custody.
Officers probing the dissident republican bombing arrested the 50-year-old for questioning about an armed robbery in the city last Tuesday.
IMAGE: The blast on Saturday night outside Bishop Street Courthouse
Monday also saw three significant security alerts in the city, triggered by the hijacking of two vehicles by masked men.
In the first incident, three men reportedly hijacked a white Transit van in the Circular Road area at around 11.30am before throwing an object in the back and abandoning the vehicle.
Army bomb disposal experts carried out a controlled explosion on the van.
Just over two hours later, at 1.45pm, police received a report that four masked men - one allegedly armed with a gun - had hijacked a postal delivery van on Southway.
Police said the two occupants of the van were ordered to drive to Lonemoor Road and leave it there.
Both incidents prompted sizeable security alerts, with nearby residents evacuated.
The third incident saw another abandoned vehicle in the city cause panic on Monday night.
Police attended Northland Road after an Asda delivery van was left parked across the road, stopping traffic in front of St Mary's secondary school.
Elderly residents were evacuated from their homes in pyjamas as police attempted to secure the area.
Police Service of Northern Ireland (PSNI) Superintendent Gordon McCalmont warned of widespread disruption.
"We understand the effect this will have on the local community, but we will not take any chances when it comes to keeping people safe," he said.
IMAGE: The damage caused to the Ford Fusion car outside Bishop Street Courthouse
On Monday afternoon, Northern Ireland Secretary Karen Bradley told MPs the weekend bomb blast, which caused no injuries, had "absolutely nothing to do with Brexit".
She said: "Nobody should try and draw any connection between what happened on Saturday night and any of the discussions we are having in this place or with our friends in Europe.
"The attack that happened on Saturday night is a result of a threat level that has been in place since before the Brexit vote.
"These are plots and activities that these people have been working on and trying to carry out for many, many years and we need to be clear with them that those activities are not welcome, the people of Northern Ireland do not want to see this kind of activity on their streets."
A dissident republican group styling itself as the New IRA has been blamed for the bombing outside Bishop Street court.
IMAGE: Forensic investigators examine the scene
Dramatic CCTV footage of the incident released by police showed a group of people walking past the car containing the bomb minutes before the explosion.
The Brexit debate has prompted claim and counter-claim about whether the imposition of a hard border in Ireland will lead to an upsurge in dissident republican attacks.
The Irish Government has warned of the prospect of an increase in violence if physical infrastructure is installed on the border, but the DUP has dismissed it as scaremongering.
Some believe the timing of Saturday's bombing may have had more to do with a symbolic anniversary in the history of militant republicanism, as it came ahead of the centenary of the outbreak of Ireland's War of Independence in 1919.
IMAGE: Smouldering remains - The aftermath of the car bomb
Ms Bradley told the Commons a "crude, unsophisticated" explosive device was used in Saturday's bomb attack and CCTV had showed "teenagers and others passing by only minutes before the device detonated".
She said: "It is sobering to think that a truly sickening outcome by those responsible was only narrowly averted.
"Those who planned this attack and who placed this crude device in a busy city centre have absolutely no regard for the people who live and work there."
DUP Westminster leader Nigel Dodds welcomed Ms Bradley's assertion that the bomb was unrelated to the Brexit debate, adding it is also "somewhat unrelated" to Stormont's collapse.
He said: "Let us not legitimise in any way what these people are about by ascribing to them a cause - these are people who want to destroy communities on both sides, who want to disrupt, who want to bring nothing positive and who have no agenda whatsoever, and therefore I totally support what the Secretary of State has said in that regard."
Ms Bradley later denied she had brought up "border polls" in the region in talks about Brexit, calling claims on the subject "rumours and supposition".