Updated: All seems to be fine at this time.
An emergency alert was sent out by MSU around 1 PM
"MSU ALERT, Man with a gun seen heading into Bessy hall secure-in-place immediately. please advise others if you can. www.msu.edu for more info"
It has since been claimed that the weapon was a training weapon and that all is well.
Since this post now exists, kudos to MSU staff for moving quickly on this and deploying alerts, as well as locking down classrooms quickly.