Det Supt Fiona McCormack, who is overseeing the investigation, said on Friday that officers had established the initial cause of the fire was a fridge-freezer and that it was not started deliberately.
She said they were trying to get to the bottom of why the fire grew so quickly and tests had pointed towards the cladding using aluminium composite tiles and the insulation behind it. Investigators will now seek to establish whether the use of these materials was illegal.
McCormack said: “Preliminary tests show the insulation samples collected from Grenfell Tower combusted soon after the tests started. The initial test on the cladding tiles also failed the safety tests.”
She added that the insulation proved “more flammable than the cladding”. McCormack said police would investigate how the tiles were installed.
“We will identify and investigate any criminal offence and, of course, given the deaths of so many people, we are considering manslaughter, as well as criminal offences and breaches of legislation and regulations,” she said.
McCormack said documents and materials had been seized from a number of organisations but no one had been questioned yet as it was too early in the investigation.
She said: “We are looking at every criminal offence from manslaughter onwards, we are looking at every health and safety and fire safety offences and we are reviewing every company at the moment involved in the building and refurbishment of Grenfell Tower.”
Crown Prosecution Service figures last year showed that 19 companies had been charged with corporate manslaughter since the law was introduced in 2007.
There were 15 guilty verdicts, with fines ranging from £50,000 to £700,000, three acquittals and one case yet to come to trial. New sentencing guidelines introduced last year were expected to lead to higher fines for larger companies convicted of the most serious regulatory breaches.
Meanwhile, the government said technical experts were undertaking urgent tests of the Hotpoint FF175BP fridge-freezer named by police as the cause of the fire and a decision on whether to order a recall would be taken shortly.
Consumers who own a FF175BP (white) or FF175BG (grey) should contact the manufacturer but the government said there was no reason for consumers to switch off their fridge-freezer. Hotpoint expressed its condolences to the victims and said it would assist in the investigation.
Police said the official death toll had risen to nine but the number presumed dead remained at 79.
McCormack said every complete body had been recovered from the building, describing it as a “very, very distressing scene”. Theresa May has warned that the death toll could yet rise further.
Describing how the investigation was proceeding, McCormack added: “We have been in Grenfell Tower, from top to bottom, last week. Next week we will be installing a lift to the outside of the building to assist. But our forensic search may not be complete until the end of the year.”
Repeating concerns from police commander Stuart Cundy about the scale of the task in identifying the victims, McCormack said: “There is a terrible reality that we may not find or identify everyone who died due to the intense heat.”
She repeated previous appeals for anyone with information about people who may have been in the building but who investigators may not know about, and said the Home Office had assured police it would not be checking people’s immigration status.
The Department for Communities and Local Government said last week that in buildings over 18 metres high, cladding “using a composite aluminium panel with a polyethylene core would be non-compliant with current building regulations guidance”. However, some experts have suggested this was not the case.
Speaking in the Commons on Thursday, the prime minister refused to say whether the cladding used in the tower was legal or not.
Thousands of tower block residents around the UK have been told that their homes are clad with the same flammable aluminium panels believed to have fuelled the blaze at Grenfell Tower.
About 600 high-rises across the UK have been clad, and some of these are likely to have flammable systems, the DCLG has estimated.
Councils have been asked to conduct safety checks, sending building materials to Whitehall to be tested.
May said the government would fund tests on up to 100 towers a day.