In this aerial image made from video, emergency workers help people to disembark a train near the Parsons Green Underground Station after an explosion in London Friday, Sept. 15, 2017. A reported explosion at a train station sent commuters stampeding in panic, injuring several people at the height of London鈥檚 morning rush hour, and police said they were investigating it as a terrorist attack. (Pool via AP)
LONDON鈥擝ritish police raced Saturday to track down whoever planted a bomb on a packed London Underground train, as authorities raised the national threat level to maximum, meaning another attack may be imminent.
Friday morning鈥檚 blast, which left 29 people injured, was Britain鈥檚 fifth terror attack in six months and was claimed by the Islamic State group.
Prime Minister Theresa May announced late Friday that the threat level had been raised to 鈥渃ritical鈥�, and said troops would take over guarding key sites to free police officers for deployment elsewhere.
The explosion at Parsons Green station in south-west London sparked a 鈥渨all of fire鈥� that left passengers with burns and caused a stampede of panicking people in which some were trampled.
No-one has yet been arrested over the bombing, but anti-terrorism police chief Mark Rowley said the investigation was making 鈥渞eally good progress鈥�.
鈥淲e鈥檙e chasing down suspects,鈥� he told reporters.
鈥淪omebody has planted this improvised explosive device on the Tube. We have to be open-minded at this stage about him and potential associates.鈥�
May said military personnel would take over guard duty at certain closed 鈥減rotected sites鈥�, freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain.
The country was last on critical alert after the bombing at a concert in Manchester in May, which was also claimed by the IS group.
In a statement Friday, the group said an IS 鈥渄etachment鈥� had carried out the London attack.
Rowley earlier said most of the injuries were due to 鈥渇lash burns鈥�, while others were wounded as passengers ran out of the station in panic.
Witness Charlie Craven told Agence France-Presse he heard a 鈥渕assive bang鈥�, adding: 鈥淚 saw an orange sort of fireball encompassing the whole Tube coming towards you.鈥�
Another, Lauren Hubbard, described it as 鈥渁 wall of fire鈥�.
Twitter user @Rrigs posted pictures of a white bucket smoldering on the train and described how a 鈥渇ireball flew down carriage and we just jumped out open door鈥�.
The bucket, which was inside a frozen food bag from the budget supermarket chain Lidl, looked like the type used by builders and there appeared to be cables coming out of it.
鈥楿nhelpful speculation鈥�
US President Donald Trump said that 鈥渓oser terrorists鈥� were behind the attack, adding that they were already 鈥渋n the sights鈥� of British police.
London鈥檚 Metropolitan Police dismissed the tweet as 鈥渦nhelpful speculation鈥�, while May also rebuked him.
鈥淚 never think it鈥檚 helpful for anybody to speculate on what is an ongoing investigation,鈥� she said, speaking after an emergency meeting of senior ministers.
May said the device was 鈥渃learly intended to cause significant harm鈥�, condemning it as a 鈥渃owardly attack鈥�.
Speaking on Friday evening, Rowley said the remnants of the bomb were being examined by forensic scientists.
British media reported that it had a timer but failed to detonate fully.
Hans Michels, a professor of chemical engineering at Imperial College, said the flash flame 鈥渟uggests that the explosion was only partly successful鈥�.
鈥淢uch of the bucket still seems to be intact and there appear to be no victims with lethal impact wounds,鈥� he said.
Armed police on patrol
Twenty-nine victims were treated in hospital, although health authorities said none were in a serious life-threatening condition, and eight were released on Friday.
Louis Hather, 21, had been travelling to work and was three carriages down from where the explosion took place.
鈥淚 could smell the burning. Like when you burn plastic,鈥� he told AFP.
He was trampled on as passengers stampeded out of the station and his leg was badly cut and bruised.
Sally Faulding, a 51-year-old teacher, said: 鈥淧eople were falling over each other.鈥�
Richard Aylmer-Hall, 52, told the Press Association: 鈥淭here was panic, lots of people shouting, screaming, lots of screaming.鈥�
The area around Parsons Green station 鈥� a quiet and wealthy residential district, filled with chic cafes 鈥� was evacuated for most of the day.
Local residents and businesses rallied together to offer tea, phone charging points, and the use of their toilets to people unable to get home.
Four previous attacks in London and Manchester this year claimed the lives of 35 people.
Three of those attacks involved a vehicle ploughing into pedestrians.
The other attack was a bombing in May at a pop concert by US star Ariana Grande in Manchester which killed 22 people, including several children.