Hundreds of passengers were stranded in a tunnel and several were left hurt when a Tube train derailed in London.
Three carriages came off the rails on the westbound Central Line between Mile End and Bethnal Green stations.
Transport for London (TfL) said 700 people were taken off the derailed train, and another stuck behind it, by about 1100 BST after a two-hour wait.
Metronet said initial reports suggest the derailment was caused by a loose "bale of material" inside the tunnel.
"Our initial reports suggest that a bale of material became dislodged from its licensed storage position in a tunnel cross-passage," said a Metronet spokesman.
The Tube maintenance firm has ordered an immediate review of all similar storage sites used by staff and contractors to be completed before the start of service on Friday morning.
Metronet said it will be investigating the cause of the incident with TfL officials.
Crying and upset
Howard Collins, of TfL, said the accident was the first derailment of a passenger train for a number of years.
A Central Line train came off the tracks at Chancery Lane in January 2003, injuring 32 passengers and causing the closure of the line for 11 weeks.
On Thursday morning, passengers were led down the track to Mile End station. A number of people suffered minor injuries, TfL said.
It said 20 people had received medical treatment, including five people who were taken to hospital with minor injuries such as breathing problems.
Chris Christofi, who was on the derailed Tube, said panic had broken out among the passengers who thought a bomb had exploded.
"We felt a massive jolt underneath the train which caused the train to move up and down and sideways.
"There were some windows that seemed to blow in and explode and there was some soot that came into the carriage.
'Uncomfortable and sweaty'
"There were a lot of people crying and upset, a lot of people falling over. We thought there'd been a bomb."
But a TfL employee in the same carriage calmed the situation down by telling everyone he thought it was a derailment.
The driver also quickly informed passengers what had happened.
Richard Porter, of the London Ambulance Service, said temperatures in the tunnel could have been around 26C (79F) to 31C (88F).
"People were uncomfortable and sweaty," he said. "The rescue operation was very swift.
"We had water and were helping people very quickly."
The train had just left Mile End station when it derailed.
The LAS attended the scene along with London Fire Brigade who sent 14 appliances, including four search and rescue vehicles.
Central Line services are suspended between Leytonstone and Liverpool Street, and there are severe delays on the rest of the line.
Mile End and Bethnal Green stations are also closed.