Manchester suicide bomber Salman Abedi carried his bomb in a Karrimor rucksack triggered with a sophisticated switch held in his left hand, leaked crime scene photographs suggest.
Police found the remains of the bomb's lightweight metal casing, together with a high quality 12-volt battery and a trigger device contained a small circuit board soldered into one end, the Daily Telegraph reported.
The evidence discovered in the foyer of the Manchester Arena helps to explain why the security services are so convinced Abedi was part of a wider terrorist cell, as the level of expertise needed to build such a device points to an expert bomb-maker who could still be at large.
Crime scene photographs were leaked to the New York Times, apparently after being shared with US intelligence agencies by British investigators.
The pictures were leaked despite a direct plea from Amber Rudd, the Home Secretary, to the US authorities to stop leaking information about the fast-moving inquiry.
The pictures show the shredded remains of a blue Karrimor rucksack lying on the tiled floor of the foyer.
The trigger switch, contained in what appears to be a brass casing, has a small circuit board and a red wire protruding from one end, which experts have suggested could point to a remote-control element built into the trigger as a failsafe.
It would have allowed an accomplice to detonate the device if Abedi lost his nerve.
Other pictures, not published by the New York Times, apparently show that nuts and screws penetrated metal doors and left deep marks in brick walls.
A diagram of where the fatalities occurred shows that they were mainly standing in a circle around the bomber, but a gap in the circle is further evidence that the bomb was in a backpack, rather than a vest, and that Abedi's body shielded those standing directly in front of him.
His upper torso was reportedly found some distance away, propelled forwards by the backpack.
The even spread of the fatalities also suggests the bomb was carefully and evenly packed.
Police found the remnants of a Yuasa 12 volt, 2.1amp lead acid battery, said to be more powerful - and, at £12, more expensive - than batteries found in other suicide bombs. The batteries - similar to a small car battery in appearance - are sometimes used to power emergency lighting.