A U.S. Army helicopter was flying higher than permitted limits when it collided in midair with an American Airlines Group Inc. regional jet last

But the day after American Airlines Flight 5342 collided with a U.S. Army helicopter in Washington, D.C., killing all 67 people onboard, shares of the company were trading just 2.5% lower. The decline grew four days later, with shares trading 4.5% lower than the day before the crash.