The defense attornies argued successfully in a MA court that since it wasn't possible to figure out which bat had killed which victim, murder was not provable.
I believe they changed their laws a bit since then, but you'd think that by the time the 20th Century had come around even MA legislators would have figured out how to describe mob or group action and how to hold members accountable for the crimes.
In many/most states, going back for ages, any death (even of another perpetrator) during the commission of felony is grounds for a murder convictions against all the perpetrators (even getaway car drivers) who where provably part of the initial felony. The courts and or laws in Massachusetts are truly demented!