The bomber in the Manchester terrorist attack appeared to have carried a powerful explosive in a lightweight metal container concealed either within a black vest or a blue Karrimor backpack, and may have held a small switch in his left hand, according to preliminary information gathered by British authorities.

Remnants of backpack

The initial analysis of the bomb, based on evidence photographed and collected at the crime scene and distributed by British authorities, does not specify the size or type of explosive used in the bomb’s main charge but suggests an improvised device made with forethought and care.

Possible switch located in suspect’s left hand

Law enforcement images of metal nuts and screws propelled by the blast, and of damage nearby, show that the bomb’s makeshift shrapnel penetrated metal doors and left deep scuffs in brick walls.

Nuts and screws used as shrapnel

And the authorities’ review of the blast site shows that many of the fatalities occurred in a nearly complete circle around the bomber, Salman Abedi, whose upper torso was heaved outside the lethal ring toward the Manchester Arena entrance.

Manchester Arena Torso of bomber Box office Doors to arena Victims Foyer Approx. location of detonation Bridge to Victoria Station Manchester Arena Torso of bomber Box office Doors to arena Victims Foyer Approx. location of detonation Bridge to Victoria Station Manchester Arena Torso of bomber Box office Doors to arena Victims Foyer Approx. location of detonation Bridge to Victoria Station The New York Times

All of these are indicators of a powerful, high-velocity charge, and of a bomb in which its shrapnel was carefully and evenly packed.

The location of the bomber’s torso, and the apparent absence of fatalities in a line between the blast site and where his remains landed, was said by one explosive disposal technician who examined the images to indicate that the explosive charge was more likely in a backpack than in a vest, and propelled the bomber away from the blast.

Manchester Arena Exit Stage Exit Exit Parking Exit Blast area People seen running down these stairs in video. Victoria Station Train tracks Manchester Arena Exit Stage Exit Exit Exit Blast area Victoria Station Train tracks The New York Times

Certain details of the bomb further suggest a desire by a bomb-maker to reduce the risk of a dud.

The authorities found a mangled Yuasa 12-volt, 2.1 amp-hour lead acid battery at the scene, which is more powerful than batteries often seen in backpack bombs or suicide vests. The battery, used for emergency lighting and other applications, can be bought for about $20.

12-volt battery that was possible power source

A possible switch to initiate the explosion, carried in the bomber’s left hand, was also unusual in a suicide device, in that it appears to have contained a small circuit board soldered inside one end.

It is not clear from the law enforcement images if the object was a simple plunger switch, or included a timer or a receiver that could be operated remotely via radio signal – or some combination, or something else.

Such redundancy, if the object was the switch, could give the bomber or a cell more than one option for deploying the device, and suggest that the bomb was not as simple in design as many terrorist devices, which often are crude and prone to failure or haphazard effect.

One independent analyst of improvised explosive devices, Michael C.L. Johnson, suggested that the object might be an electronic cigarette and unrelated to the bomb’s detonation – an understandable case of investigators focusing on a crime-scene detail early in a case.