He turned to see a plume of white smoke rise, when suddenly another explosion followed, doubling the smoke.

WSJ: Runners Recount Chaotic Scene at Finish Line of Boston Marathon

The blaze was discovered at 16:50 BST by a police officer who noticed "a large amount of smoke and heard an explosion on some wasteland near to the travellers site".

BBC: Gas explosion at Paices Hill travellers site

He heard the first explosion and saw the smoke just as he crossed the 26-mile mark.

NPR: The Finish Line: What The Bombing Was Like

"I had just run to the finish line and like 30 seconds later I heard the first explosion, and then turned around and saw the smoke, " he said.

NPR: Marathon Runner Witnesses Double Disasters

Television footage earlier in the day showed heavy smoke and flames rising from the home and witnesses reported hearing an explosion.

WSJ: FDNY investigates partial house collapse in Queens

Mrs Philpotts' mother, who lived with the family, said she had heard an explosion and the sound of broken glass but had been unable to reach her daughter because of thick smoke.

BBC: House fire scene