Mark Haddock is being held at Antrim police station
Loyalist leader Mark Haddock has been arrested in connection with four murders dating back to the 1990s.
He is being questioned about the murders of John Harbinson in May 1997, Gary Convie and Eamon Fox in March 1994 and Sharon McKenna in January 1993.
Haddock was arrested by officers from the Historical Enquiries Team.
Police said the 40-year-old, who is being held at Antrim Police Station, is also being questioned about a number of assaults and a shooting incident.
The senior UVF figure is currently serving a 10-year sentence in Maghaberry prison for attacking a club doorman.
Earlier this year, Haddock was charged with the murder of rival loyalist, UDA leader Tommy English, in 2000.
A spokesman said a total of 16 people had now been arrested under Operation Ballast. He said 11 of those had been charged and four were the subject of reports to the PPS.
Operation Ballast was launched after former Police Ombudsman Nuala O'Loan's investigation into allegations of collusion between RUC officers and the Mount Vernon unit of the UVF in north Belfast.
The investigation was triggered on the back of a complaint by Raymond McCord snr, the father of a UVF victim Raymond McCord jnr, who had claimed no-one had been prosecuted for his son's killing in order to protect Haddock.
In October 2005, the Irish Labour TD Pat Rabbitte, using parliamentary privilege, named Haddock as a British Special Branch agent and a serial killer.
He said his victims included the four killings he is now being questioned about.
They are: John Harbinson who was murdered on 18 May 1997. He died after being handcuffed and beaten by a UVF gang on the Mount Vernon estate in north Belfast.
Sharon McKenna, a Catholic woman, who was murdered on 17 January 1993. Described as a "Good Samaritan", she was shot dead while visiting a Protestant pensioner at his north Belfast home. Eamon Fox and Gary Convie, Catholics builders from County Armagh, were shot dead on 17 May 1994. They were eating their lunch in a car parked close to a building site on North Queen Street in the Tiger's Bay area of north Belfast when they were attacked.