In this task, you are given an article. Your task is to summarize the article in a sentence.
Q: The 25-year-old died in a booby-trap car bomb in Omagh, County Tyrone, on 2 April 2011. No-one has been convicted.
On the fourth anniversary of his murder, Det Ch Insp Una Jennings issued a fresh appeal for information.
She said there were "considerable challenges remaining in this lengthy and complicated investigation".
"Although a significant amount of progress has been made, the key to putting people before a court charged with Ronan's murder lies with local communities in Omagh and east Tyrone," she said.
"From the early days of the investigation, we said this was going to be a long-term inquiry with Ronan's murder at its core but incorporating several linked incidents, including attempts to murder other police officers, a bomb attack, arms finds and armed robberies.
"It has proved to be an extremely large investigation but one which still has the potential to be concluded by charging suspects."
The detective said the investigation into Constable Kerr's murder and a series of linked incidents had "generated a substantial amount of investigative activity":
"Ronan's murder is as devastating for his family today as it was four years ago," she said.
"Their pain remains so devastating that they feel unable to talk publicly about their plight on the fourth anniversary."
Mr Kerr, who was a Catholic and an active member of the GAA (Gaelic Athletic Association), was seen by many as a symbol of Northern Ireland's new police service.
He was the second officer to have been murdered by paramilitaries since the Royal Ulster Constabulary became the PSNI in 2001.
Dissident republican paramilitaries have been blamed for the killing.
A: 
The key to catching the killers of Constable Ronan Kerr lies with local communities, the detective in charge of the murder investigation has said.