WASHINGTON (AP) — 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 in an outbreak of politically loaded menace with little if any precedent. Authorities warned there might well be more.

Law enforcement official...