Authorities later announce that they recovered a pressure-cooker bomb after the pursuit into Watertown, a source briefed on the ongoing investigation said.

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The police also later found the pressure-cooker bomb at the gunfire scene.

WSJ: Relief in Boston as New Details Emerge in Hunt for Suspected Bombers

And technology yielded the first major break: Police found photos and even a surveillance video of the two suspects toting backpacks -- each ostensibly loaded with a pressure-cooker bomb -- at the marathon's home stretch.

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If its domestic stockpile could in this way be kept below a bomb's worth, this would also ease pressure in Israel for a military strike on Iran's nuclear facilities and so buy yet more time for talks.

ECONOMIST: Iran pockets Bushehr and plays on