Police are trying to understand what "invisible handcuffs" were on the three women allegedly kept in slavery at a house in Lambeth, Scotland Yard said on Friday.
The two suspects bailed in connection with the investigation into slavery, a 67-year-old man and a 67-year-old woman, were previously arrested in the 1970s, Scotland Yard told reporters.
The three women, a 30-year-old British woman, a 57-year-old Irishwoman and a 69-year-old Malaysian woman, are not thought to be victims of trafficking. The 30-year-old is reported to have spent her entire life in captivity.
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."
Journalists heard that the victims were allowed out of the house "in carefully controlled circumstances".
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".
People have "no right to be sceptical" about this case, police added.
Detective Inspector Kevin Hyland said the whole of the human trafficking unit - 37 officers - are now working on this investigation.
Specially-trained officers are working with the women to try to understand their lives over the last 30 years or more, he said.
DI Hyland said the women are in the care of a specialist non-governmental organisation.
"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.
DI Hyland said the two suspects have also been arrested on suspicion of immigration offences.
He was not prepared to disclose the nationalities of the two people arrested, but said they have been in the country for "many years".
Police do not believe the victims were trafficked into the UK, he said.
DI Hyland said the police search of the address in south London took 12 hours, and said they seized 55 bags of evidence amounting to in excess of 2,500 exhibits.
Police said the Malaysian and Irish Embassies have been contacted.
Investigating officers said on Thursday night they had "never seen anything of this magnitude before".
Aneeta Prem, Freedom Charity founder, said on Thursday that the alleged victims - who are believed to have suffered physical and mental harm - were able to walk out of the property after extensive calls with the charity.