Crash investigators have concluded preliminarily that improper calibration of an airspeed sensor during maintenance touched off the sequence of events that led to October’s fatal Lion Air jetliner crash in Indonesia, according to people familiar with the details.

The conclusion is subject to further analysis, these people said, but it is the firmest indication so far that a suspected maintenance lapse was the initial misstep that ended with the months-old Boeing Co. 737 MAX aircraft plunging into the Java Sea, killing all...