Two men, aged 17 and 37, have been arrested by detectives investigating the murder of Constable Stephen Carroll, police said today.

Constable Carroll, 48, was gunned down last night as he responded to a call for help from a frightened woman who said her house was being attacked.

He was shot from behind as he got out of his car, investigators have revealed today.

The officer was struck in the head by a single bullet that passed through his back window.

It is understood that Constable Carroll was due to retire from the force next year.

Dissident republican group, the Continuity IRA, said they were behind the the murder.

Both of those arrested are being questioned in Antrim police station.

In a coded message, they said the shooting in Craigavon was carried out by their north Armagh battalion.

The message said: "As long as there is British involvement in Ireland, these attacks will continue."

The killing followed the murder of soldiers Mark Quinsey, 23, and Patrick Azimkar, 21, outside the Massareene Barracks in Antrim on Saturday.

Meanwhile, Northern Ireland’s First and Deputy First Minister today called off their trade mission to America a second time following the Craigavon killing.

Peter Robinson said: “Our future isn't dependent on the evil of those who seek to destroy our society, but rather it depends on the good that there's found in those throughout our community who want to build a stable, peaceful and democratic society,” he told MLAs.

Deputy First Minister McGuinness, a former IRA commander, described the dissidents as "traitors to the island of Ireland."

He called for the traditional code of silence to be broken and for tips to be passed to police.

"I want to join with Peter Robinson to wholeheartedly appeal to everyone, and anyone, who has any information whatsoever about these killings, to pass that information to the police, north and south," said McGuinness.

"We need to pledge our support to Hugh Orde."

Sinn Fein president Gerry Adams said there was no support whatsoever in the republican or nationalist communities for what happened and his party would support the apprehension of those involved.

In a show of solidarity, they were supported by the Ulster Unionist, SDLP, Alliance, PUP and Green Party leaders.