Police confirmed that a purple liquid was poured through the letterbox of 21 Stanley Road, South Harrow,between 9-11am on the day of the explosion, May 7.
The liquid had the smell of rotten eggs and experts are being consulted to establish what this substance was.
The house is now inaccessible and the damage to the property means it will take some time to find and analyse the liquid.
Colin Sutton, chief inspector in charge of homicide and special crime said: "We believe a substance was put through the letterbox of number 21 which was purple in colour and liquid in form.
"There are a number of theories about what it could be and we are working with chemists and scientists to try and establish what it is.
"It is going to take us some time to gather the necessary forensic evidence because of the complete devastation."
The explosion is believed to have taken place at number 21 and an emergency call was made to the police at 10.50am on May 7, where the caller reported a group of girls causing a disturbance outside the house.
However, during the conversation the caller said that the girls had moved away and she did not need police assistance.