Why pressure cookers make such deadly explosive devices

A suspicious device found in New York City may be an improvised explosive device made from a household pressure cooker, according to early reports from local law enforcement. 
Police retrieved the device on Saturday night on West 27th Street in Manhattan's Chelsea neighborhood. Nearby, on West 23rd Street, an explosion injured 29 people, all of whom have since left the hospital, NYPD confirmed Sunday.
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