A box found in the wreckage of a bus and marked "microwave" began a security alert which delayed the emergency services after the 7/7 attack.
The box, which officers feared might be a secondary device, was found stored in a luggage rack of the bus which was destroyed on Tavistock Square when Hasib Hussain blew himself up, killing 13 people on board.
It was brought on board by Gladys Wundowa, 50, from Ilford, Essex, and was harmless. But its presence led to a search of the bus with sniffer dogs – a task which lasted several minutes and was conducted while horrifically injured passengers lay dying. Ms Wundowa, who worked as a cleaner at University College London, was among those killed by the blast. The inquest also heard how the explosion sent a massive "chimney" of grey smoke billowing into the air, laden with debris and human body parts.
The thick plume soared up to 200ft above the red double-decker. As the dust settled, police officers who had witnessed the explosion distinguished a pair of legs protruding from the wreckage, while desperate victims cried out for help.
Recalling the scene, British Transport Police (BTP) Constable Gary Sims said: "There was a very dark, grey-white smoke cloud which went possibly 100ft to 200ft up in the air and as that went up... there was a lot of debris and there were unfortunately parts of people in that.
"At the same time the two sides came out from the rear of the bus. Areas came out and curled towards us and at that point the roof came off." This landed in front of the bus, he told the 7/7 inquest at the Royal Courts of Justice in London. His colleague, BTP Inspector Ciaran Dermody, told how the shocked pair, along with fellow officer PC Neville Lazenby, watched from an unmarked police car as the top of the bus appeared to open out before them and thick smoke rose high above it.
"It went up out of the bus almost like a chimney," he said. The roof then "flopped" down. "We all in the car, I think, said at the same time, 'There's a bomb on the bus'," he said. "We knew instantly it was a bomb; we didn't think it was anything else.
"The bus seemed to be compressed down to my height, which is 6ft 1in," said Insp Dermody. "There were a number of people on that top deck of the bus trapped."