The survivors of the Miami Showband attack have urged soldiers who suspected their ranks had been infiltrated by loyalists to help shed light on the UVF atrocity.
Three members of the popular cabaret band were shot dead by a UVF gang after being flagged down as they drove to Dublin after performing in Banbridge, Co Down in July 1975.
Those killed were lead singer Fran O'Toole, 29, guitarist Tony Geraghty, 23, and trumpet player Brian McCoy, 33.
The killers opened fire after two of its members blew themselves up as they attempted to plant a bomb inside the band's bus.
The gang included a number of rogue Ulster Defence Regiment soldiers. It had flagged down the bus after setting up a fake UDR security check point on the A1 in Co Down.
The two band members who survived the massacre - Des Lee and Stephen Travers - are currently taking legal action against the PSNI and British Ministry of Defence.
Their claim, which also involves the widows of Mr O'Toole and Mr McCoy, focuses on the contention that the UDR did not do enough to prevent collusion with loyalists at the time.
Days after the 40th anniversary of the attack, they have appealed for former UDR soldiers to help their quest for justice.
Michael Flanigan, the Belfast-based solicitor representing the survivors and bereaved families, said: "Even now, 40 years later, the event is still as shocking as on July 31 1975.
"It is well established that by the time the attack on the Miami Showband took place, the UDR had experienced years of infiltration by, and regular loss of weapons to, loyalist paramilitaries.
"We are seeking to take statements from former UDR members about their experience within the UDR from this period.
"We are satisfied that many people who joined the UDR would have been aware of loyalist infiltration and concerned about its impact on the regiment.
"In particular, we would like to hear from former UDR members who may have reported concerns about collusion to their superiors within the regiment."