Pressure cooker bombs used in Boston blasts

The Boston bombs were lethal devices consisting of metal, nails and ball bearings crammed into pressure cookers, it has emerged.

According to US sources, both bombs, packed with the deadly shards, were carefully placed in black duffel bags and hidden near the finishing line. The twin explosions have already claimed the lives of three, injuring over 140 others.

They had been deliberately designed to cause explosions that would injure, maim and kill innocent bystanders, the sources said.

The two explosions came on the Patriots' Day holiday, 12 seconds apart.

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The first, just metres from the finish line, rocked the final stretch of the marathon route four hours into the race on Monday.

There then followed a second explosion about 100 metres back on the same street.

Over 17,000 competitors had completed the run before the bombs were detonated.

Meanwhile, US President Barack Obama on Tuesday admitted the US authorities still had no idea who was behind the attacks.

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