A man in his 80s has been arrested in connection with the 1985 killing of Gérard Hoarau, an exiled politician shot several times with a submachine gun on his doorstep.
The suspect was detained by Metropolitan police counter-terrorism officers as he arrived at Gatwick on a flight on Thursday.
He remains in custody at a central London police station on suspicion of conspiracy to murder.
Hoarau was killed aged 34 in Edgware, north London, on 29 November 1985 with a Sterling submachine gun.
He was the leader of Mouvement Pour La Resistance, an opposition party in Seychelles, and had been living in exile in the UK capital.
Last week, an arrest was made in connection with his death. The first suspect arrested, who was also held on suspicion of conspiracy to murder, was released pending further inquiries.
Police said the 77-year-old man, who was detained in Antrim, Northern Ireland, had not previously been arrested as part of the investigation.
A Met spokesman said the force was unable to provide further details about the second suspect, such as whether he had previously been arrested, due to operational reasons.
In 1986, three people were convicted of perverting the course of justice by meddling with the investigation.
The trail went cold for decades, prompting a review in 2016 from which fresh lines of inquiry were established, the Met said.