A second man, who was arrested as part of a garda investigation into the murder of Denis Donaldson in Co Donegal in April 2006, has been released without charge.
A file is being sent to the Director of Public Prosecutions.
Two men, aged in their 40s and 30s, were arrested on Sunday night.
A man in his 40s was also released without charge yesterday.
Mr Donaldson, a senior Sinn Féin official, was shot dead at an isolated cottage near Glenties in April 2006.
The 55-year-old had been living there since his exposure as an MI5 agent the previous year.
In 2009, dissident republican group the Real IRA claimed responsibility for the killing.
