Swab tests were carried out on a lamppost against which the gunman stumbled after Constable Jim Doherty (43) was hit.

It is understood the force of the shotgun blast knocked him off his feet and an accomplice had to help him to his feet.

Dissident republicans have been blamed for the shooting in Bishop Street.

Hundreds of children and parents were in the area at the time Constable Doherty's car was blasted. He was hit in the back, arm and face by pellets.

His condition is described as "comfortable".

Officers returned to the scene as part of the investigation and interviewed at least 50 motorists and pedestrians.

Constable Doherty, a Catholic from Derry's Bogside but who was based in Omagh, Co Tyrone, joined the PSNI two years ago.

The gunman struck seconds after the son got out of the officer's blue Peugeot 607. He was slowing down as he approached traffic lights when the single shotgun blast was fired through a rear passenger window. Constable Doherty was hit in the arm, face and back.

The gunman then fell backwards, stumbled against the lamppost before being pulled to his feet by his accomplice.

The two then ran off, got into a black BMW car which raced off towards the city's Leckey Road flyover towards the Creggan estate where it was set alight.

Police believe the two men may have tried to disguise their appearances in some way.

Thursday's attempted murder has shocked all sides in Ulster. Politicians claim it was a deliberate attempt to intimidate Catholics joining the PSNI.

Since a new recruitment policy was introduced in November 2001, the number of Catholics among the 7,500 officers has risen rom 10% to just over 23%.

The investigation is being headed by DCI Frankie Taylor who said co-operation from the public had been very good, but added: "There are people out there that we know have still to come forward."