Northern Ireland's Deputy First Minister Martin McGuinness has hit out at the police investigation into the dissident republican murder of Catholic officer Ronan Kerr.

The Sinn Fein representative was among the senior figures who led cross-community condemnation of the killing in April.

But in an unusual step he has now criticised the probe and questioned the arrest of a 22-year-old man in Coalisland, Co Tyrone, who has since been released by police.

Mr McGuinness said: "I have been in contact with the PSNI (Police Service of Northern Ireland) at the top level to register my dissatisfaction and annoyance at the operation in Coalisland. The nature of the operation has caused widespread anger in the local community. That is a reality the PSNI will now have to face."

In a response the PSNI said it carried out its investigations to the highest standards "without fear or favour in accordance with the law".

Constable Kerr was killed when a booby-trap bomb exploded under his car outside his home in Omagh, Co Tyrone, on April 2.

Constable Kerr's murder sparked an unprecedented display of political unity against the violence. Mr McGuinness was prominent as the leaders of unionism and mainstream republicanism stood shoulder-to-shoulder with the police at the young officer's funeral.

The Deputy First Minister said: "People are in no doubt where I stand in relation to the killers of Ronan Kerr. I believe that the PSNI have a duty to properly investigate this murder. However, the arrest and release of this young man raises very serious questions about the quality of the current PSNI investigation. Bad policing operations in republican communities will do nothing to bring his killers to justice. Indeed if anything the opposite is the case."

But a spokesperson for the PSNI said: "This investigation is protracted and wide-ranging, encompassing the murder of constable Kerr and associated incidents. It is being conducted to the highest professional standards. Police have a duty to follow all lines of inquiry without fear or favour in accordance with the law."

Police later said the remaining four men arrested on Tuesday had been released. The 23-year-old woman detained by detectives remains in custody.