LONDONDERRY, Northern Ireland (Reuters) - Northern Ireland police said the three security alerts they were examining in Londonderry on Monday have ended after the department confirmed the incidents, including one where they carried out a controlled explosion, to be hoaxes.

Police had carried out a controlled explosion on a vehicle on Monday that was hijacked by masked men in Londonderry and examined two other abandoned vans, two days after a car bomb had 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.

"We can confirm that all three alerts are hoaxes, however, we cannot underestimate the impact these incidents have had on our community," police said here early on Tuesday.

The department’s statement on Twitter did not provide much details.

Officers had evacuated homes and cordoned off both areas as they inspected the vehicles.

People have now returned to their respective homes.

A Royal Mail postal van was hijacked nearby later on Monday by four masked men, one of whom was reported to have a gun, police said. A third vehicle, which a Reuters witness identified as an Asda supermarket delivery van, was abandoned in another part of the city a few hours later.

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There had also been an attempted hijacking of a local bus elsewhere, police said.

No one was injured in the blast outside a court 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.

The blast came at a time when police in Northern Ireland and European Union member Ireland have warned that a return to a hard border between the two after Brexit, complete with customs and other checks, could be a target for militants.

However, neither Britain’s Northern Ireland minister, Karen Bradley, nor the Police Service of Northern Ireland (PSNI) said they saw any links between Brexit and the incidents in the province’s second-largest city, which lies close to the border with Ireland.

“We are not picking up any information that indicates that anybody wants to engage in violence in relation to the Brexit issue, certainly not at this point,” PSNI Assistant Chief Constable Mark Hamilton told Irish broadcaster RTE.

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Bradley said the threat level in Northern Ireland would remain at “severe,” where it has stood since 2009 when two British soldiers and a policeman were killed in the worst attacks in the province for over a decade.

“Although there has been a reduction in the overall number of national security attacks in recent years, vigilance in the face of this continuing threat remains essential,” she told Britain’s parliament in London.

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A fifth man, aged 50, was arrested on Monday in relation to Saturday’s attack under the Terrorism Act. The four other men detained earlier in connection with the incident were released without charge, police said.

There were no details from police on who may have been behind the hijackings in Londonderry, also known as Derry, particularly among Catholics to show their resistance to British rule.

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

The New IRA 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 one leadership.

“It’s an organization that has evolved over recent years,” the PSNI’s Hamilton said. “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.”