Police were questioning three men last night over the murder of a top loyalist, Bobby Moffett, in a packed shopping street.
Moffett, understood to be a member of the Red Hand Commando (RHC) terror group, was gunned down in Belfast's Shankill Road in front of dozens of shocked bystanders.
Police arrested a 35-year-old man tonight in connection with the killing.
Detectives were already questioning two men aged 25 and 31 who were both arrested earlier today.
A fourth man aged 49, arrested on Friday night, was released this morning.
All the men were arrested in the Belfast area.
Moffett, 43, was shot a number of times in the face by two masked gunmen in an assassination blamed on renegade elements within the Ulster Volunteer Force (UVF) - a mainstream paramilitary group with historic ties to the RHCs.
It is understood Friday's murder was linked to a personal dispute.
Police described the shooting as "a cold-blooded, ruthless killing".