The investigation into the murder of PSNI officer Ronan Kerr is continuing to prove testing nearly a year after the young officer’s death, the Chair of a staff association has said.

Terry Spence, of the Police Federation of Northern Ireland, said that while there had been arrests in connection with the probe it was always going to be a challenge.

PSNI Cons Kerr (25) died on April 2 2011 in Omagh after a booby-trapped bomb exploded under his car. He had just left his apartment to go to work in Enniskillen.

The PSNI are now focusing their enquiries on 11 unidentified vehicles and an unidentified man in the days leading up to the murder and the day itself.

The investigation into the murder of PSNI Cons Kerr and a linked series of incidents is one of the most extensive since the Omagh bombing in 1998, a statement from the Force said.

Mr Spence emphasised: “I think there are a lot of complex issues surrounding this case. It has proven to be difficult to apprehend the offenders to date.

“The police have been very proactive in trying to get evidence – and there have been a significant number of arrests. Regrettably there have been no charges brought.”

Mr Spence said homes had been searched and vehicles examined during the investigation.

Meanwhile, Det Supt Raymond Murray from the Serious Crime Branch, said the vehicles and a man they are looking to identify could prove vital to the probe.

He added: “We are ten months into this investigation but there is a long way to go.

“We have trawled through an enormous amount of information obtained from our enquiries and material gleaned from previous appeals.

“But there are still gaps which we are endeavouring to close in terms of building up a complete picture of the events leading up to the explosion.”

Police have released a map detailing the whereabouts of the vehicles in the hope it will bring further leads.

To date, there have been 56 searches, 10 arrests and one person charged in connection with a wider overall investigation. The charge relates to a linked incident.

Additionally a comprehensive forensic strategy is being progressed involving hundreds of exhibits submitted for examination at laboratories in Great Britain and the USA.

House-to-house enquiries are also continuing in Omagh.