Police are investigating a suspected car bomb explosion outside a courthouse in the the city of Derry in the northwest of Northern Ireland on Saturday, January 19.

Police from Derry City and Strabane District tweeted an image that appeared to show a vehicle on fire at the side of a road, saying that that Bishop Street in Derry – also known as Londonderry – Northern Ireland’s second-largest city, was closed due to a “suspected car bomb.”

Bishop Street Closed STAY AWAY Suspected Car Bomb pic.twitter.com/S3cFu3zy0i — PSNI DC&S District (@PSNIDCSDistrict) January 19, 2019

No one is believed to have been injured, the police said on Facebook.

The BBC reported that the incident occurred at around 8:15 p.m. Police had received a warning and were evacuating nearby buildings including a hotel when the the vehicle exploded.

The Irish Times reported that police received a warning around 30-minutes before the explosion. It said that a pizza delivery van hijacked earlier in the day in the Brandywell area was understood to have been used in the attack.

Shortly after the blast, Belfast Telegraph journalist Leona O’Neill tweeted video of the scene, later describing the vehicle as a van.

The smouldering remains of what looks like a transit van outside Derry’s court house in Bishop Street. Bars and clubs in the area have all been evacuated pic.twitter.com/ouINZRat7w — Leona O'Neill (@LeonaONeill1) January 19, 2019

Police said a second vehicle in Bishop Street was “causing suspicion,” and that a number of bars and homes were evacuated, Q Radio’s David Hunter reported. Police later said on Facebook that vehicle had been “checked,” and that officers were dealing with “an extensive crime scene.”

LATEST pictures of tonight's car bomb in Derry – Londonderry Bishop Street area. By @McAuleySteven . @LeonaONeill1 is also at the scene and we'll have the full story on @qnewsdesk . A major police operation is continuing in the city, where another vehicle is still being examined. pic.twitter.com/CWAnhjAuFc — David Hunter (@davidhunter7) January 19, 2019

No-one has claimed responsibility for the blast.

Update January 20 The Police Service of Northern Ireland issued a statement about the incident.

“At around 7.55 p.m. last night officers on patrol in Bishop Street spotted a suspicious vehicle and were making checks when, around 5 minutes later, information was received that a device had been left at the courthouse,” Assistant Chief Constable Mark Hamilton said.

“We moved immediately to begin evacuating people from nearby buildings including hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club. The device detonated at 8.10pm,”

“At this stage it appears as though the vehicle used had been hijacked from a delivery driver in the Quarry Street area a short time before the explosion.”

“Two men, both in their twenties, have been arrested by detectives investigating explosion outside Bishop Street courthouse in Derry/Londonderry,” Derry City and Strabane District police tweeted.

Hamilton later told journalists that the main line of enquiry is the New IRA.

He said that a pizza delivery van was hijacked by two armed men at about 6 p.m. The delivery driver was held after his vehicle was stolen to transport the bomb which was left outside the courthouse at 7:23 p.m.

Three minutes later, the Samaritans in the West Midlands of England received a telephone warning.

The bomb, which Hamilton described as a “highly unstable, crude device that could have detonated at any time,” went off at 8:09 p.m.

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

Police later arrested another two men in the city.

In a Saturday news release, Saoradh, a political party formed in 2016 by dissident Irish republicans opposed to the peace process in Northern Ireland, said that reports indicated that “a British institution” was attacked “in what is believed to have been a large mine attack by Republican Revolutionaries.”

Saoradh said that the incident came on the eve of the centenary of the Soloheadbeg ambush in which two Royal Irish Constabulary officers were killed, an action seen by some as the first in the Irish War of Independence that eventually led to the 1921 partition of Ireland which created Northern Ireland. The ambush took place on January 21, but Saoradh organised a commemorative event for January 20.

The 1998 Good Friday or Belfast Agreement ended what is known as The Troubles, three decades of violence in Northern Ireland beginning in the late 1960s in which more than 3,500 people were killed, the majority by predominantly Catholic Irish republicans who want the reunification of Ireland, but also by Protestant loyalists who want Northern Ireland to remain part of the United Kingdom, as well as the security forces.

The violence also spilled over into Ireland, the United Kingdom mainland, and Europe.

Some paramilitary actors oppose the peace process that sprang from the agreement, and there have been sporadic violent incidents since, but car bombings are rare.

Police in Northern Ireland and Ireland have said that a return to a hard border on the island after Brexit could result in an increase in attacks by militant groups.

The largest dissident Irish republican paramilitary group is known as the New IRA. It was formed in 2012 after a merger of several smaller groups with the Real IRA.

In February 2010, Newry courthouse was targeted in a car bomb attack that was linked by prosecutors to the Real IRA.

The Real IRA was also blamed when a beer keg filled with around 50 kg (100 lb) of explosives was left in a stolen car was abandoned near the Bishop Street courthouse in March 2011. The bomb, described by police as a “substantial viable device,” was destroyed in a controlled explosion. It apparently contained ammonium-nitrate-based fertiliser and sugar, as well as as PETN and RDX.

The New IRA is believed to be responsible for a number of small bomb attacks in Derry in 2015, and the group claimed responsibility for a spate of Derry gun and “grenade” attacks in July 2018. Some have linked the New IRA to Saoradh, but there is no explicit connection between the two.

In June 2018, Assistant Chief Constable Stephen Martin told The Irish Times that the New IRA was “the most significant threat” in Derry, but he said that membership of various groupings is fluid and can change frequently.

Martin said the majority in leadership roles were active during the Troubles in the Provisional IRA, by far the largest republican paramilitary group, or the INLA. Both groups have ended their armed campaigns.

Martin added that the threat posed was “small compared to what the Provisional IRA would have posed during the Troubles, but in today’s context it is a severe threat and we don’t take it lightly.”

“Their ambition is not matched by their capability and their capacity,” he said.

In 2018, the numbers of paramilitary-style attacks by all dissident republican groups decreased compared to the previous year. The number of bombings fell from 29 to 17, according to the PSNI.

On August 3, 2010, Óglaigh na hÉireann, another Irish republican dissident group, claimed it had detonated a 200-lb car bomb outside the Police Service of Northern Ireland station on Strand Road in Derry.

In separate incidents in November 2013, masked men ordered a van driver and a bus driver to deliver suspect packages to Strand Road police station, a tactic that is colloquially known as a proxy bomb. Those incidents have also been attributed to ONH.

ONH announced a ceasefire in January 2018, although some members reportedly formed a new paramilitary group.

This post was updated multiple times on January 19 and 20.