According to the BBC:
Three of the devices found were the same size and weight as those used for the 7 July London bombings, while the fourth was smaller and appeared to have been contained in a plastic box. The same chemicals appear to have been used.
Scotland Yard Assistant Commissioner Andy Hayman told the news conference: “At this stage it is believed the devices consisted of homemade explosives and were contained in dark coloured bags or rucksacks. It is too early to tell how these were detonated.”
BBC security correspondent Gordon Corera told BBC Radio 4’s Today programme the devices were so similar there was speculation they could have been part of the same batch.
“The explosive might have degraded over time or had not been put together right in this case, or it could have been a completely different batch of explosives – homemade – that had not been cooked up properly.”
The bombers’ plan might have been disrupted by the investigation into the 7 July attacks, forcing them to act before they were fully prepared, Mr Corera added.
Of course, none of the options are good. Coordinated attacks mean a larger terror network. Uncoordinated attacks suggest a process of mimetic emulation. While the former might be more dangerous, it would also be, in theory, easier to disrupt.