LONDON — Police in Northern Ireland said Sunday they suspected Irish Republican Army dissidents to be responsible for a “reckless” car bombing outside a courthouse in the city of Londonderry.

Two men in their 20s have been arrested over the explosion which happened shortly after 8.00 p.m. local time (3.00 p.m. ET) on Saturday as people were socializing in nearby venues.

The attack comes at a time when there are concerns for what Britain’s looming departure from the European Union will mean for the border between Northern Ireland, which is part of the U.K., and the Irish Republic, which is a separate country and will remain part of the E.U.

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The border is currently more or less invisible and there are no checkpoints. Some fear the reinstatement of a physical boundary risks a return to a bloody 30-year-conflict known as "The Troubles," rekindling tensions that might spill over into violence. A 1998 peace deal ended decades of conflict.

The police said Sunday that they had received a tip off at around 8.00 p.m. that a device had been left at the courthouse in the center of the city. Officers began evacuating people from nearby buildings including hundreds of hotel guests and a large number of children from a nearby church youth club.

Around 10 minutes later a bomb exploded in a delivery vehicle parked outside the building. There were no reports of injuries. "This attack was unbelievably reckless,” Assistant Chief Constable Mark Hamilton said in a statement released Sunday.

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

"Fortunately it didn't kill anybody but clearly it was a very significant attempt to kill people here in this community," he later said at a news conference. The police later released CCTV footage from before the attack which showed a group of young people walk past the vehicle on Bishop Street.

Hamilton said currently the “main line of inquiry” was that the bomb had been planted by a dissident republican group known as the New IRA. "The New IRA, like most dissident republican groups in Northern Ireland, are small, largely unrepresentative, and just determined to drag people back to somewhere they don’t want to be," he added.

Some 3,600 were killed in during the so-called Troubles, which pitted republicans, primarily Catholics seeking a united Ireland, against security forces tasked with maintaining British rule over Northern Ireland. The republicans also fought against local loyalists, primarily Protestants wanting Northern Ireland to remain in the United Kingdom. Groups on all sides were among the perpetrators and victims.

A fragile peace was negotiated in 1998 and since then most militants have renounced violence. The last fatal attack involving a car bomb was carried out in 2016 by the New IRA when a prison officer was fatally injured by a bomb left under his vehicle in Belfast.

Lawmakers from all sides — including Sinn Fein, the former political wing of the IRA — have condemned the attack. "Shame on you. Shame on you and stop," said Mary Lou McDonald, leader of Sinn Fein which signed the peace agreement. She told the BBC that the blast was an "outrageous attack."