HELENA — The failure of two quarter-inch bolts were the probable cause of a June plane crash that left one Helena Valley resident with minor injuries, according to investigators.

Mahler’s plane didn’t respond. A controller asked the plane to turn left. More silence. That time period correlates with the reports of the electrical failure on board.

In the aftermath of last week’s Alaska Airlines in-flight emergency, some aviation experts are questioning the structural design of the section of the Boeing 737 Max 9 that blew off the plane.