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Police investigating the attempted murder of two police officers in County Tyrone in Northern Ireland have arrested four men.

The officers had been on patrol in Strabane when a bomb detonated at the side of their vehicle on Thursday.

Neither officer was injured in the explosion, which happened shortly before 23:00 GMT in the Mount Carmel Heights area.

Police arrested three men - two aged 36 and one aged 28 - on Friday.

They also conducted a number of searches.

A fourth man, aged 38, was arrested on Saturday.

Detectives earlier said a strong line of inquiry was that dissident republican group the New IRA was behind the attack.

Earlier on Friday, PSNI Assistant Chief Constable Bobby Singleton said the incident caused a major security alert in the area.

It affected more than 1,000 residents and left some children unable to get to school.

Police have said they discovered what could be a command wire for the improvised explosive device during follow-up searches.

Mr Singleton said the officers, who had been conducting routine patrols over ongoing antisocial behaviour in the area, were left "shaken" when they saw a flash and heard a loud bang.

"They left the area, came back to the station and they found evidence of some blast damage to their police vehicle," he added.

The Secretary of State for Northern Ireland Chris Heaton-Harris expressed relief that no-one was injured.

"I condemn those involved in this attempted attack and reiterate my support for the officers concerned," he tweeted.

PSNI Chief Constable Simon Byrne also condemned the attack, describing it as "senseless".

Sinn Féin deputy leader Michelle O'Neill said the attack was "reprehensible and reckless" while Democratic Unionist Party (DUP) leader Jeffrey Donaldson said it was clear that those responsible offered "nothing but hurt and disruption for the communities they purport to represent".