The two 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 whether they were convicted at the time – adding that the current investigation “will take considerable time”.
The pair have been arrested on suspicion of immigration offences as well as in connection with the investigation into slavery and domestic servitude, Scotland Yard said.
The victims – a 30-year-old British woman, a 57-year-old Irish woman 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, London, last month.
It is thought that the 30-year-old woman had been in servitude all her life.
Police said that over many decades the suspects and the victims would “probably have come into contact with public services”, including the Metropolitan Police, who arrested the suspects in the 1970s.
The case came to light after the “very distressed” Irish woman rang a charity to say she had been held against her will in a house in London for more than 30 years.
Police would not reveal the nationality of the two suspects but said they have been in the country for “many years”.
They 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”.
Mr Rodhouse said: “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.”
He added: “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.”
No more details on those arrests were given.
He said the investigation will take “some considerable time”, and there are a number of lines of inquiry to follow up, numerous statements to take and lots of exhibits to examine.
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 police are trying to understand “what were the invisible handcuffs being used to exert such a degree of control over these women”. He added that to label the investigation as domestic servitude or forced labour is “far too simplistic”.
Detective Inspector Kevin Hyland, from the human trafficking unit (HTU), said the whole of the unit – 37 officers – are working on this investigation.
“Whilst we do not believe that they have been subjected to sexual abuse, we know that there has been physical abuse, described as beatings.
“However, there is nothing to suggest that the suspects were violent to others outside of the address,” he said.
Mr Hyland said the two suspects have also been arrested on suspicion of immigration offences.
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 have been bailed until a date in January.
Police would not go into any detail about whereabouts in Ireland – north or south – the Irish woman is from.
Journalists heard the victims were allowed out of the house “in carefully controlled circumstances”.
Aneeta Prem, founder of the Freedom Charity, said the Irish woman contacted them after seeing a piece on the news she had done about forced marriage and dishonour violence.
The woman had also seen a number of interviews Ms Prem had given throughout the summer about the issue of forced marriage.
Ms Prem said they initially received “the most basic detail, guarded detail” of what was happening, and then over the course of a week the charity gained the women’s trust.
She added: “Considering the horrendous circumstances they’ve been in, they’re doing remarkably well.
“This is the start of a very, very long journey.”