Photographs of a burning Lidl bag holding a plastic container have emerged following an explosion at Parsons Green station.
The unverified pictures showed wires trailing from the bag, which was discovered on the District Line train.
The Metropolitan Police said "several people" were injured at the west London station and said it was treating the explosion as a "terrorist incident".
:: Follow the latest updates on The Independent's liveblog
Witnesses described flames and a "fireball" in the train carriage at about 8.20am today.
Sylvain Pennec, a software developer from Southfields, near Wimbledon, was around 10 metres from the source of the explosion.
"I heard a boom and when I looked there were flames all around," he told the BBC. "People started to run but we were lucky to be stopping at Parsons Green as the door started to open."
He described the scene of panic as commuters struggled to escape the carriage, "collapsing and pushing" each other.
Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.
"It looked like a bucket of mayonnaise," he said. "I'm not sure if it was a chemical reaction or something else, but it looked home made. I'm not an expert though."
Police evacuated all homes within 100m of the underground station in the wake of the blast.
The Aethrieus Temple school, close to the station, was placed on lockdown with pupils kept in a hall.
