Experts contacted by BuzzFeed News say the apparent failure of the suspected device involved in the Parsons Green terror incident suggest an “amateur” attacker who may not have had direct contact with terror networks – but say these type of attacks are particularly difficult to prevent.

One former MI5 counter-terror officer said the attack appeared to follow the pattern of some earlier, failed, attacks.

“My instinct says this is likely to be an ‘amateur extremist’ similar to Exeter Giraffe cafe bomber who was radicalised online and set fire to his leg with a viable but badly made device some years ago,” he said.

These types of attacker are though, he added, very difficult to trace in advance because they don’t need much contact with others, often don’t travel overseas, and don’t always need to purchase materials that raise red flags.

“Unless the agencies get lucky and the attacker makes contact with a low level source, possibly even a special branch source as they try and get some material or brag about what they are going to do there is no way to identify them [in advance],” he said.

“The media often ask this irresponsible question of ‘why didn’t the agencies know about him?’. Ask yourself, if you decided tomorrow, with no criminal history or certainly nothing related to terrorism, to go to a library and google make a bomb then follow the open source online instructions would anyone know about it? No, not if you didn’t tell them.”

Another terror expert, Brian Painter of Discreet Security, told BuzzFeed News that home-made devices based on internet guides are often unreliable and prone to failure.

“These build your own bomb guides often found on extremist sites are notoriously unreliable and ineffective, however it does not deter people from attempting to make them,” he said. “The items used in homemade explosives are over the counter items that when purchased in small quantities are virtually impossible to detect that they are going to be used for sinister means.

“It’s important that if anyone suspects that someone is making purchases, acting strangely, becoming radicalised in any way, that the authorities are made aware and let them determine the threat, if any.”

– James Ball