The Lugones couple were probably killed by CORU . . . the anti-Castro terrorist group based in Miami that assassinated Orlando Letelier in September 1976 on behalf of DINA (Chile/Pinochet). CORU's leader Orlando Bosch was in Buenos Aires in 1975, soliciting business from AAA (Argentine Anticommunist Alliance). The Lugones couple were political rivals of Admiral Massera, who also ordered hits on personal enemies in Paris and elsewhere. CORU also helped recruit U.S. mercenaries for Angola; they probably signed up Lonnie Henry, who apparently was a paratrooper just back from 'Nam, and who likely only had one little part in a highly compartmentalized plot. These Bay of Pigs vets were all CIA trained (at Fort Benning, Fort Jackson, Fort Bragg) and quite sophisticated; in fact Luis Posada Carriles was head of Venezuela's state security agency at one point. These guys did hundreds of terroristic bombings---for which they claimed credit---and who knows how many covert acts. They were also protected by U.S. Republicans, particularly the Bush family, which made extensive FBI cases against them go away.
In short, CORU had known connections to both Argentina's junta, which likely ordered the hit, and N.C. mercenaries, such as the one found with the murder weapon.
Good luck getting any law enforcement activity on this, after the FBI had to throw out reams of evidence against these guys. President George H.W. Bush actually pardoned Bosch in 1990. There is no official agency investigating this case anymore. The Sumter County sheriff doesn't even have a file on it. The Argentines aren't going to want to admit that Scilingo lied (re a baby being born at ESMA detention center) since his testimony has been involved in so many cases. There simply isn't any political will to prosecute this, not at this late date. Even if there were, what good would it do the families? They know their children were killed by the junta.