Pressure cookers, he said, help build up pressure when using "low explosives" those less powerful than a more sophisticated explosive like C-4.
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2.
The marathon bombs, the affidavit said, were "low-grade explosives that were housed in pressure cookers" and packed with BBs, nails, and green-colored hobby fuse.
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3.
According to investigators, the devices were rudimentary: low-grade explosives made more powerful by being inside pressure cookers, with nails attached to maximize their lethality.