By Maria Cramer and Brian R. Ballou, Globe Staff

A Suffolk Superior Court jury this afternoon was unable to reach verdicts on the murder charges against the alleged triggerman in the Mattapan massacre case and acquitted the man who was accused of being the killer’s getaway driver.

The jury pondering one of the most horrific crimes in recent Boston history was unable to reach verdicts on four counts of first-degree murder and five other counts against Dwayne Moore in the September 2010 slaying of four people, including a 2-year-old boy. The jury acquitted Moore of one count of cocaine trafficking.

After seven days of deliberations, the jury acquitted Edward Washington of four counts of first-degree murder and five other charges.

The courtroom briefly erupted into chaos as the clerk read the jury’s verdicts on the nine charges against Washington and some people swore out loud before being pushed out of the courtroom by court officers...