Two men have been arrested under the Terrorism Act in connection with the murder of Constable Ronan Kerr in April 2011.
A 27-year-old man was arrested in Omagh and a 40-year-old man was produced into police custody from prison.
They have both been taken to Musgrave’s Serious Crime Suite where they are currently being questioned by police.
PSNI officer Ronan Kerr (25) died after a booby-trap bomb exploded under his car as he left his apartment in Highfield Close at 3.45pm on Saturday, April 2, 2011.
He was the second serving police officer to be killed since the Good Friday Agreement, following the murder of Constable Stephen Carroll in 2009.
Dissident republicans opposed to the peace process calling themselves the New IRA have been blamed for the attack on the Catholic new recruit.
Constable Kerr was dubbed a "modern-day hero" by then PSNI chief constable Matt Baggott.
He had joined the police in May 2010.