Bomb seen in past failed attacks: experts

The improvised bucket bomb left on a London Underground train bears the hallmarks of previous failed bombing attempts in the UK, experts say.

Images of the explosive shared on social media appear to show a bucket placed in a Lidl shopping bag with wires protruding, and reports suggest the device also had a timer.

Academics and chemical experts suggested that, while the device caused what witnesses described as a "fireball", it failed to fully detonate.

Dr Lewis Herrington, a lecturer and terrorism expert at Loughborough University, said the use of a timer was "key", as it distinguished it from the suicide attacks on July 7, 2005 and at the Manchester Arena earlier this year.

"A timing device is critical - if you look at all the other attacks ... they all wanted to die," he said.

"That's not present and that really sends alarm bells.

"This guy really wanted to pack a punch.

"The initiator succeeded hence the reported fireball but clearly the explosive element failed."

Chemistry expert Professor Hans Michels said there were similarities with the failed attempts to bomb the underground on July 21, 2005.

"In appearance and arrangement the remnants of the device seem highly similar to those of the hydrogen peroxide-based devices of 2005," the professor at London's Imperial College said.

"The size of the device and its containment in a plastic bucket is also the same.

"The flash flame reported suggests that the 'explosion' was only partly successful. In particular much of the bucket still seems to be intact and there appear to be no victims with lethal impact wounds.

"I must speculate that either the mixture was not of the right composition or that the ignition system was inadequate or not properly placed. The fact that much of the bucket is intact supports the latter possibility."

The trial of the July 21 bombers heard that the second set of bombs failed because the bombers had got the chemical recipe wrong.