WASHINGTON — Investigators searched coast-to-coast Thursday for the culprit and motives behind the bizarre mail-bomb plot aimed at critics of the president, analyzing the innards of the crude devices to reveal whether they were intended to detonate or simply sow fear two weeks before Election Day.

Three more devices were linked to the plot — two addressed to former Vice President Joe Biden and one to actor Robert De Niro — bringing the total to 10. Authorities warned there might be more.

Law enforcement officials told The Associated Press that the devices, containing timers and batteries, were not rigged like booby-trapped package bombs that would explode upon opening. But they were still uncertain whether the devices were poorly designed or never intended to explode. A search of a postal database suggested at least some may have been mailed from Florida. Investigators are homing in on a postal facility in Opa-locka, Fla.

Details suggested a pattern — that the items were packaged in manila envelopes, addressed to prominent Trump critics and carried U.S. postage stamps.

The list of bombing targets spread from New York, Delaware and Washington, D.C., to Florida and California.