Anti-terror police are examining the bucket bomb which failed to fully detonate at Parsons Green as they attempt to catch the bomber.
Photos of the explosive show a Lidl coolbag with a large builder’s tub inside and a string of Christmas tree lights hanging over the top.
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 home made.’
Terrorists have attempted to use Christmas tree lights as detonators to bombs before, although experts say this device was ‘very crude’.
Photos of the bomb at Parsons Green show a builder’s bucket inside a Lidl coolbag with apparent fairy lights hanging out of it
Pictures on social media show a bucket alight on the underground train after the explosion
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 to set rags, cloths on fire.’
He said it may include a chemicals, such as ammonia nitrate, to attempt to cause an explosion.
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, so at the moment you have to keep an open mind.
‘That said of course, the most devastating land-based terror attack in Europe, in Madrid, had a similar modus operandi, so the MO could suggest something.
He added: ‘It’s not a high-explosive that functioned because the blast and shockwave would have killed multiple people.’
Pictures from the District Line train appear to show a burning plastic bucket stashed in a Lidl carrier bag, which exploded
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
He said experts would wait to see whether ISIS or any other terror group claimed responsibility for the device.
If fairy lights were used, it would 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.
Transport for London said Tube services were suspended between Edgware Road and Wimbledon
Chief Superintendent Matt Ward, of West Midlands Police, said of Hussain’s bomb at the time: ‘Forensic examination of the pressure cooker found it contained mixed urea, nails, drills bits, nuts, bolts, steel sockets and diesel fuel.
‘The Christmas tree lights also found at his address had been adapted so each one could be used as an explosive initiator or detonator.
‘Examination of his computer discovered browsing history relating to terrorism including bomb making and bomb blast injuries, showing he had a consistent and continued interest in terrorism and conflict.’