Letters sent by the bureau’s Seattle office are a sign that the Justice Department’s investigation into Boeing, the maker of the plane whose fuselage panel blew off, is ramping up.

Passengers aboard an Alaska Airlines plane that made an emergency landing after a fuselage panel blew off this year have started to receive letters from the F.B.I. identifying them as possible victims of a crime.

The letters are a sign that a criminal investigation the Justice Department has opened into Boeing, the manufacturer of the 737 Max 9 jet, is ramping up.