Metropolitan Police Assistant Commissioner Mark Rowley has said 'hundreds' of officers are currently 'trawling through CCTV' in a desperate bid to close in on the 'suspects' involved in the Parsons Green bomb.

Rowley said Met Police had embarked on a complex investigation and while the device had now been safely defused, his officers were also starting to examine some of the pictures sent in from witnesses at the scene.

The Metropolitan Police Assistant Commissioner told reporters: 'We are making excellent progress at the moment as we pursue our lines of inquiry to identify, locate and arrest those responsible.

'We have hundreds of police officers trawling through CCTV footage, detectives have spoken to tens of witnesses and we have taken a large number of calls from to the hotline... for members of the public.

'Indeed members of the public have sent in so far 77 images and videos of the scene... which are now being assessed for evidential value.'

It comes after Theresa May raised Britain's terror threat level from critical to severe, meaning another attack on the public could be imminent.

More armed police will be deployed to the streets while soldiers will take over a number of security functions such as manning power stations.

Meanwhile he confirmed the improvised explosive of the device has now been made safe and has now been taken away for special forensic examination.

Photos of the bomb at Parsons Green show a builder's bucket inside a Lidl coolbag with apparent fairy lights hanging out of it. It is thought the bomb failed to fully detonate

Rowley said Met Police had embarked on a complex investigation and 'hundreds' of officers were 'trawling through CCTV footage'

Explosives experts say the Parsons Green bomb could have killed dozens if it had properly detonated at rush-hour this morning.

Anti-terror police are now examining the bucket bomb which failed to fully go off on a tube train in west London as they try to track down the bomber.

Photos of the explosive show a large builder's tub - believed be packed with explosives - inside a Lidl coolbag, with a string of Christmas tree lights, thought to have been a crude detonator, hanging over the top.

Officers compared the device to those used in the failed 21/7 bombings, in which jihadis attempted to blow up tube trains. In that case, the bombs' detonators went off but the explosives themselves were not ignited.

Pictures on social media show a bucket alight on the underground train after the explosion. It is understood to have had a timer, meaning it could have been intended for a busier station

It is believed the bomb was left on the busy district line train from Wimbledon this morning before the bomber escaped. Police are analysing the device to track down the terrorist

ISIS have encouraged followers to use the lights in bomb guides they post online.

An ex-Army bomb disposal officer said he believed the bomb could have been the Mother of Satan explosive used in the failed 21/7 terror plot.

Chris Hunter said the bombmaker 'almost certainly' used triacetone triperoxiede (TATP), a sensitive and highly explosive material used by jihadists.

Mr Hunter told Sky News: 'When I saw the bucket and when I saw it burning, it looked exactly like the 21/7 device.'

The Mother of Satan has been used in several terrorist attacks, including 7/7 and the Manchester Arena bombing.

He added: 'There are very few chemical compounds readily available to jihadists that can be used to make a high explosive so the main charge will also certainly be TATP.

'This substance is also known as the Mother of Satan. It is super sensitive just after being made and can go off very easily. However, it doesn't remain sensitive for very long, it deteriorates quickly.'

In that instance, a group of Islamic terrorists planned to carry out a copy-cat attack to 7/7, but although the detonator part of the bombs went off, they did not ignite the explosives and only one person was injured.

A timer was also found on the Parsons Green device, suggesting the terrorist left it on board the train and then got off. It also suggests it was intended to go off at busier stations further down the line.

There were also reports that the bucket had been filled with nails and was to be set off by a booster charge of hexamethylene triperoxide diamine.

Terror expert Will Geddes said: 'My suspicion is that Parsons Green was not the intended target.

'Unless a person knows it, it is not going to mean a great deal to you - if it was Paddington or Notting Hill, they are internationally recognised names.'

'I think it was more a premature detonation than anything else. This could have been a lot worse. It could have killed a significant number of people.'

Others say the bomb probably didn't detonated properly because it was badly made.

Explosives expert Dr Sidney Alford told MailOnline: 'The fact that an initial bang was heard but the bomb did not even shatter the plastic bucket it was contained in suggests to me this could have been hydrogen peroxide.

'I can rule out other types of explosives as if the initial bang was heard with those they would have gone off completely and destroyed the bucket.'

Dr Alford said the bomb properly did not go off because it was poorly designed.

Witnesses reported a 'strong, acrid chemical smell' in the carriage after the explosion.

Pictures from the District Line train appear to show a burning plastic bucket stashed in a Lidl carrier bag, which exploded

Specialist forensic officer, including some with expertise in dealing with bombings and chemical incidents, are combing the train for clues

A victim, believed to be a schoolchild, is carried from the station with charred legs as others were treated for burns in the street and neighbouring shops

A traumatised and injured passenger on the ill-fated Parsons Green train is taken away by paramedics and firefighters

Major General Chip Chapman told Sky News: 'This doesn't look like a high-end explosive from ISIS such as TATP (triacetone triperoxide) or, if it was, it failed significantly, the booster or detonator didn't go off.

'That said of course, the most devastating land-based terror attack in Europe, in Madrid, had a similar modus operandi.'

He added: 'It's not a high explosive that functioned because the blast and shockwave would have killed multiple people.'

Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit

Transport for London said Tube services were suspended between Edgware Road and Wimbledon

Examining the image of the bomb, chemical weapons expert Hamish de Bretton-Gordon told MailOnline: 'It looks pretty unsophisticated.

'It could be an incendiary device with some sort of detonator in a big white plastic bucket, perhaps with some sort of chemical like ammonia nitrate, fertiliser bomb to cause an explosion.'

He said the attack looked like 'classic jihadi terrorism' and the bomb could have been 'devastating' if it had detonated properly.

Mr de Bretton-Gordon said the attack would raise further questions over whether the sale of certain chemicals should be more heavily regulated.'

Describing the device, witness Sylvain Pennec said: 'It looked like a bucket of mayonnaise. I'm not sure if it was a chemical reaction or something else, but it looked homemade.'

The incident has been compared to the 21/7 attacks in which bombs were placed on the underground and the detonators went off, but did not ignite the explosives

In that instance, the bombers Muktar Said Ibrahim (left) and Ramzi Mohammed were tracked down by police and convicted of terror offences

This was one of the failed bombs in the 21/7 attacks, with explosive substances left on the floor of a London bus when the device failed to properly detonate

It is not be the first time extremists have used so-called them to build a device.

In May this year, a radicalised former doorman Zahid Hussain was found to have built an explosive device in his bedroom with fairy lights, shrapnel and a pressure cooker.

He is said to have researched bomb-making techniques online, with police finding a wealth of notes and instructions at his home in Birmingham when he was arrested in 2015.

Similarities between the device and that used in the Boston Marathon bombing in 2013 have also been drawn.

In that case, timers were also used to allow the bombers to get away. However, in that case the explosives in a pressure cooker.