That or the mag spring was shot.
Or the trooper was mistaken. Other stories, such as the one on Fox, indicate that Pastor Winters was shot 3 times, and 4 others were also shot, and that 2 had knife wounds. They report 5 others, in addtion to the Pastor and the killer, transported to the hospital.
If all that's true, the killer may have just shot the gun dry. Typical .45 has 7 round magazine, and if the magazine was not topped off after loading, then that's 7 rounds available. Rather than jamming, the slide may have locked back on an empty magazine.
It should all get sorted out shortly. At minimum they should have already counted the shell casing(s) lying about.