The two suspects bailed after three women were alleged to have been held as slaves for 30 years were previously arrested in the 1970s.
They 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 case came to light after one victim told a charity she had been held against her will in a house in London for more than 30 years.
The victims – a 30-year-old British woman, a 57-year-old Irish woman and a 69-year-old Malaysian woman – are being looked after in a safe location.
Police said the Malaysian and Irish Embassies have been contacted.
Journalists heard that the victims were allowed out of the house “in carefully controlled circumstances”
Police said the two suspects have been in the country for “many years”, and said the case “so far is unique to us”.
It was described as a “complicated and disturbing picture of emotional control over many years”.
The women were rescued from a house in Lambeth in south London last month after one of them saw Aneeta Prem, founder of the Freedom Charity, on TV and contacted her charity for help.
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”.
He 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.
“That’s something we need to examine fully.”
He then said: “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 referred to 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 said that to label the investigation as domestic servitude or forced labour is “far too simplistic”.