An explosion at the three-storey block of flats was reported shortly after 17:00 GMT
An explosion in a block of flats may have been caused by the preparation of drugs, the Met Police has said.
Residents suggested the flat in Hornchurch, east London, was a cannabis factory but the force said it did not believe that to be the case.
A spokesman said a "possible cause" of the blast on Monday was the preparation of other types of drugs.
Two men have been arrested over the explosion and remain in hospital where they are receiving treatment.
The explosion caused widespread disruption to London commuters travelling to Essex on Monday.
It occurred at Bridge Point on Ardleigh Green Road shortly after 17:00 GMT with debris causing the closure of the A127 Southend Arterial Road.
Firefighters said they rescued 25 people as residents were evacuated from the building.
The cause of the explosion is not yet known
London Ambulance Service said four people had been treated at the scene and taken to hospital, while a woman and a child trapped on the building's second floor had escaped down a fire brigade ladder.
A Havering Council spokesman said the explosion happened in a ground-floor flat.
He said 41 properties had been evacuated and most residents went to a nearby restaurant while alternative accommodation was found.
Three ambulance crews, 10 fire engines and the Metropolitan Police were at the scene
