Federal, state and local investigators in New York, Washington, Florida and Los Angeles have all joined forces in the rapidly widening case, which has not yet resulted in any injuries but has sent a shock through the nation’s political and news media establishments. One central question in the inquiry is whether the bombs were the work of one person or multiple people.

Though investigators initially believed that some of the packages were delivered by hand or by courier, they have now concluded that all 10 were likely sent through the mail, a person briefed on the matter said.

Investigators are now focusing on Florida because an analysis of information collected by the United States Postal Service indicated that many of the packages were mailed from the state.

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