LONDONDERRY, Northern Ireland — Northern Ireland police cordoned off two areas in Londonderry on Monday to examine two separate hijacked vehicles for potential security threats, two days after a car bomb exploded in the city.

There was a large bang and black smoke from the first van after an army bomb disposal robot entered. Police said the van had been hijacked by three masked men who threw an object in the back before abandoning it on a residential street.

Another delivery van was hijacked nearby later on Monday by four masked men, one of them reported to have a gun, police said. Officers were evacuating a number of homes, and photographs tweeted by local reporters showed a Royal Mail van in the middle of an empty road.

No one was injured in the blast outside the courthouse on Saturday but the incident highlighted the threat still posed by militant groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province.

“Sadly, within two hours today, we are dealing with a second security alert in the city, which means even more disruption for the local community,” a spokesman for the Police Service of Northern Ireland said in a statement.

PSNI Assistant Chief Constable Mark Hamilton told Irish broadcaster RTE that it was too early to say if the incidents were linked to Saturday’s bomb, which also involved a hijacked vehicle.

A fifth man was arrested on Monday in relation to Saturday’s attack. The 50-year-old was detained under the Terrorism Act while four other men remained in custody, police said.

The main focus of the car bomb investigation is the New IRA — one of a small number of groups opposed to the 1998 Good Friday agreement.

This mostly ended a conflict between Protestant unionists who want Northern Ireland to remain part of the United Kingdom and predominantly Catholic nationalists, in which some 3,600 people had died.

The group was formed in 2012 after three of the four main militant nationalist groups merged.

It was the first time since the peace deal that most of the disparate nationalist groups still intent on violence had come together under a single leadership.

“It’s an organization that has evolved over recent years,” Hamilton said. “It has been involved in other attacks that we believe are attributed to them, such as the shooting of a police officer in Belfast and the murder of two prison officers.”

“They are a smallish grouping and they have a different presence in different parts of the province but, they remain committed to the aims of the violent dissident republican groups that we know exist here.” — Reuters