The houses will have to be demolished

She is still being treated in the hospital's burns unit, where she was said to be "doing well".

The Baileys' home has now been virtually demolished so investigators can begin the process of testing gas pressures and sifting through the wreckage.

It will be a slow process as the teams are working in an environment where safety remains an ongoing concern.

Once the police and the Health and Safety Executive have finished their initial enquiries, they will each send a report to the procurator fiscal.

Meanwhile, Mrs Leonard's son Alan Simpson said he wanted answers from gas supply company Transco.

I want some answers from Transco because in this day and age I don't think this should be going on

Alan Simpson

He said: "When I saw the state of my mother I couldn't believe it. As far as I know her hair was on fire.

"She tried to get out of the house and her hands were all burned.

"Just as she got to the door the whole house went down and she ended up in the rubble.

"Where do you go from here? I want some answers from Transco because in this day and age I don't think this should be going on."

It emerged at a news conference on Monday that Mrs Leonard's home was checked for gas leaks 18 months ago.

There have also been four call-outs to the area in the last year, a revelation which contradicted earlier statements made by the company.

Pipe 'fractured'

Spokesman John McIntosh said: "These have all been internal escapes.

"Three of them were found to be a fault in an appliance. In one there was no gas found when we were called out."

Linfield Street resident James Payne said: "The gas people are always being called out and what they've said today is just a joke. It is time for Transco to come clean."

Residents of the street have also claimed that, although a new central heating system had been installed in the houses three years ago, the main had fractured a short time later.

They insist that only the damaged part was replaced, not the whole pipe.

John McIntosh: Four call-outs

They want a full investigation into the work the company carried out.

Divisional fire officer Danny O'Donnell said the fire brigade was working with Tayside Police, Dundee City Council and the Health and Safety Executive to establish the cause of the explosion.

He said it was not possible to confirm the seat of the explosion or whether a gas appliance within the house was responsible for it.

However, he did reveal that Mrs Leonard's house at 23 Linfield Street had a gas supply and was not all electric.

Mr O'Donnell said: "We have not concluded our investigation. There was a severe explosion and a subsequent fire but we are not prepared at this stage to say what caused it.

"There's a possibility that there has been a fire and then an explosion or there's been an explosion that's caused a fire."