When a state trooper appeared at Rachel Zimmerman's door to report that her husband had jumped to his death off a nearby bridge, she fell to her knees, unable to fully absorb the news. How could the man she married, a devoted father and robotics professor at MIT, have committed such a violent act? How would she explain this to her young daughters? And could she have stopped him? A longtime journalist, she probed obsessively, believing answers would...