Homes were evacuated after two four-storey town houses worth millions of pounds collapsed in west London.
A 25m (82ft) cordon was put in place and about 40 people had to leave nearby properties in Durham Place, Chelsea, while drone teams and police dogs searched the rubble.
Emergency crews were called at 23:35 GMT on Monday after the buildings, which were being redeveloped, fell in.
No injuries have been reported, London Fire Brigade said.
The collapsed buildings form part of a terrace of seven houses that was built in the late 1700s, opposite land owned by the Royal Hospital Chelsea – the home of the Chelsea Pensioners.
A seven-bedroom house in the block sold for £16m last year, according to property website Rightmove.
A man, aged in his 30s, who lives in the area and asked to remain anonymous, witnessed the collapse.
He said it had been “extremely loud” with dust being kicked up “everywhere”.
“There were a lot of people coming out of their homes in the surrounding area to see what was going on… It was quite bad, really surreal,” he said.
According to neighbours, renovations were being carried out at the block, including in the basements.
Kensington and Chelsea Council had approved an extension to be built on the lower ground floor of the buildings in 2018.
LFB Station Commander Jason Jones said there had been “a total collapse of the buildings from the roof to ground level”.
He added: “Nobody is thought to have been inside the building at the time of the collapse.”
The Met Police said those living in nearby houses had been evacuated “as a precaution”.
Among the people asked to leave was a caretaker, who lives at the end of the road.
The woman, who did not want to be named, said she “jumped out of bed and ran down the stairs” after she heard police saying “come on, you’ve got to get out”.
“We stayed at a family friend’s,” she added.
An emergency road closure remains in place on Ormonde Gate.
A council spokesperson said neighbouring residents had been allowed back in their homes at 03:00 after safety checks had been carried out.
“The reason for the collapse is being investigated by the Health and Safety Executive,” the spokesperson said.
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