A high school reunion ended in tragedy after an Illinois man allegedly shot and killed his ex-wife and her new boyfriend in front of an estimated 100 witnesses.

Victims Lori Moore, 33, and boyfriend Lance Griffel, 36, were attending Moore's 15th year high school reunion at an East Peoria, Illinois sports bar Saturday night, according to the Pekin Daily Times.

Moore's ex-husband Jason Moore reportedly entered The Fifth Quarter Sports Bar and Pizzeria around 8 p.m. and shot his ex-wife and her boyfriend in the head at "point-blank range," according to a local NBC affiliate.

The shooting ended when an off-duty FBI officer fired at Jason Moore, fatally striking him, the Peoria Journal-Star reports.

East Peoria Police Chief Dick Ganschow credited the off-duty officer -- who asked news outlets not to identify him -- with averting a larger-scale shooting.

“It’s very difficult to say," Ganschow told the Journal-Star. "You can play the ‘what if’ game over and over again, but I think it’s pretty clear in his case the presence of this officer and his ability to take very quick and very decisive action prevented a further tragedy.”

Police are classifying the shooting as a domestic violence-related incident.

"Domestic situations are extremely volatile, whether they end up in this kind of situation or your average domestic. There is a certain level of volatility and unpredictability that are involved. The emotion level is high and sometimes they just reach a point and people snap," Ganschow told NBC.