Could be a higher powered rifle, just didn't have a loud noise on the videos I saw, also I'm not sure the proximity of the shooter and idk whether or not a suppressor was used.  A high powered rifle even at quite a distance usually is immediately recognizable as nothing other than gunfire 

 They said they found an AR. Mics are directional and have features to clip loudness. Shot was from 152 yards.  

 OK, thanks for the info, do you know what optics the shooter was using?