All five men arrested in connection with the murder of Catholic police officer Ronan Kerr in Co. Tyrone were released last night.
A 23 year old woman remains in police custody.
Northern Ireland's Deputy First Minister Martin McGuinness has hit out at the investigation into the dissident republican murder of Constable Kerr.
The Sinn Fein representative was among the senior figures who led cross-community condemnation of the killing in April.
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 on April 2nd.