The suspects bailed after three women were alleged to have been held as slaves for 30 years were previously arrested in the 1970s.
Police would not say why the man and woman, both 67, were arrested, or if they were convicted at the time — adding that the current investigation “will take considerable time”.
Scotland Yard said the pair were arrested on suspicion of immigration offences as well as in connection with the investigation into slavery and domestic servitude.
The victims — a 57-year-old Irishwoman, a 30-year-old Briton, and a 69-year-old Malaysian woman — are in the care of a specialist non-governmental organisation after they were rescued from a house in Lambeth, south London, last month. It is thought the 30-year-old had been in servitude all her life.
Police said that, over many decades, the suspects and victims would “probably have come into contact with public services”, including the Metropolitan Police, who arrested the suspects in the 1970s.
Police said the case “so far is unique to us” and it was a “complicated and disturbing picture of emotional control over many years”, in which the women were restrained by “invisible handcuffs”.
At a briefing at Scotland Yard, Commander Steve Rodhouse said police are “unpicking a story that spans at least 30 years of these women’s lives”.
He said that, to the outside world, they may have appeared to have been a “normal family”.
“This does mean that, over the course of many decades, the people at the heart of this investigation, and the victims, would probably have come into contact with public services, including our own. That’s something we need to examine fully.
“What I can say with some certainty is that the two suspects in this case were arrested by the Metropolitan Police in the 1970s, some considerable time ago.”
Mr Rodhouse said police do not believe the case falls into the category of sexual exploitation or what is traditionally understood as human trafficking.
“It is not as brutally obvious as women being physically restrained inside an address and not being allowed to leave,” he said.
He said that are trying to understand “what were the invisible handcuffs being used to exert such a degree of control over these women”.
He said that to label the investigation as domestic servitude or forced labour is “far too simplistic”.
Detective Inspector Kevin Hyland said the two suspects have also been arrested on suspicion of immigration offences. Police do not believe the victims were trafficked into the UK, he said.
The police search of the address in south London took 12 hours, and they seized 55 bags of evidence amounting to more than 2,500 exhibits.
Police said the Malaysian and Irish embassies have been contacted, and a bail condition for the two suspects is that they are not to return to the property where they were arrested. They are bailed until a date in January.
Police would not go into any detail about whereabouts in Ireland the Irishwoman is from. Journalists heard the victims were allowed out of the house “in carefully controlled circumstances”.
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