The state trooper who fired a rifle during last weekend’s Interstate 93 melee has been suspended without pay as officials determine whether or not he wrote a string of offensive online posts in recent years.Trooper Matthew Sheehan is the focus of an Internal Affairs probe, where officials say they are determining whether Sheehan authored posts under the screen name “Big Irish” on an online forum, which disparaged minority communities and discussed the use of force on the job.