The second man aged in his 30s arrested as part of a Garda investigation into the murder of Denis Donaldson has been released without charge.
A file will be prepared for the Director of Public Prosecutions.
Mr. Donaldson a former Sinn Fein member, was shot dead in April 2006 in a remote cottage outside Glenties.
His killing came four months after he confessed to working as a British agent for police and MI5 for 20 years.
Investigating Gardaí are continuing to appeal to anyone with information in relation to this murder to contact them at the Incident Room at Letterkenny Garda Station on 074 9167100, the Garda Confidential Line at 1800 666111 or any Garda Station.