The north-west London house at the centre of a mystery explosion was the target of a vandalism attack hours earlier when a purple liquid was poured through the letterbox, police said.
The incident happened about 10 hours before the property in Stanley Road, South Harrow, was levelled in the blast.
Detective Chief Inspector Colin Sutton said the terraced home is at the centre of the investigation into the explosion, which destro
yed two other houses.
Speaking at the scene on Friday, DCI Sutton said a 17-year-old woman who lived in a flat in the property was the likely target of any attack.
One resident called police earlier that day to complain about a gang of teenage girls gathered outside.
The 17-year-old girl was pulled from the rubble of number 21 Stanley Road by neighbours after the explosion on Wednesday night.
The teenager, named locally as Charlotte, was taken to Chelsea and Westminster Hospital with severe burns.
Her next-door neighbour, 26-year-old businessman Emad Qureshi, was killed by the blast shortly after 9.30pm.
Mr Sutton said: "We still haven't ruled out a gas explosion but experts say it is unlikely to be the cause.
"What we can say is that we are happy there is no link to any terrorist organisation or acts here."
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