Four men were arrested in on Tuesday by police investigating historical acts of terrorism.
One was detained for questioning about a murder in Belfast 20 years ago.
Three others aged 42, 43 and 44, were held in east Tyrone as part of a separate inquiry involving republicans, but later released without charge.
Joseph Reynolds, 40, a Catholic from west Belfast, was on his way to the Shorts aircraft factory in east Belfaast where he worked as a sub-contracting painter, when his van was ambushed in Sydenham Road by loyalist UVF gunmen on October 12 1993.
Detectives from the PSNI serious crime squad arrested a 50-year-old man on Tuesday in connection with that shooting.