The Massachusetts transit officer who was shot while responding to Friday's shooting that killed a Massachusetts Institute of Technology police officer—and sparked the citywide manhunt for the Boston Marathon bombing suspects—went into cardiac arrest and lost his entire blood supply but is expected to recover, doctors say.

Richard Donohue Jr., a 33-year-old member of the Massachusetts Bay Transportation Authority, was shot in the right thigh during a shootout with the suspects in Cambridge, Mass., late Friday. A bullet severed Donahue's femoral vein and artery, Dr. David Miller, a critical care physician at Mount Auburn Hospital, said on Sunday, and he began to "bleed out."

He went into cardiac arrest at the scene, and it took doctors 45 minutes to fully revive him, the hospital said.

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