A neighbour heard a "mighty bang" and said the roof "came off" in an explosion and fire at a house in which a man died and a woman suffered serious injuries.
Bedfordshire Police said "increased" levels of gas had been found in the area which were believed to be naturally-occurring from an underground source, not from a gas pipe network.
Emergency services were called to the house in Cleat Hill, Bedford, at about 07:00 BST on Saturday.
A neighbour said: "I just heard the bang, I heard a mighty bang that was really massive - I've never heard anything like it."
The "whole roof" of the house "came off" in the explosion, she said.
"We weren't allowed to go closer because it wasn't safe."
She added that she had been told to call 101 to ask for updates on when it would be safe to return to her home.
Gas levels
Ch Supt Zara Brown, of Bedfordshire Police, said: "Following the explosion and the subsequent identification of increased levels of natural gas in the area, an evacuation of the immediate area has been undertaken and a cordon remains in place as a precaution.
"The gas levels identified are thought to be naturally-occurring gases from an underground source and not coming from a gas pipe network in the area."
She said she appreciated home owners wanted to get back into their properties, and thanked them for their patience and understanding.
"Safety is our main priority so we are asking people to avoid the Cleat Hill area while the investigation continues," she added.
"Our thoughts remain with the family and friends of the person who sadly died, and the person being treated in hospital.
"We remain firmly in the response phase as we continue to make both the structure safe and maintain on-scene gas monitoring."
Bedfordshire Borough Council set up an emergency assistance centre for people who had to leave their homes.
Emergency services, gas workers and the Health and Safety Executive were on site on Sunday.
Some residents returned to collect belongings.