Big-Bully-Brother

Gunner woke up one morning last week and went from being the sweet big brother to a mean MONSTER! All of a sudden he is attacking Karlee every chance he gets. If she walks by him and he doesn't like it he'll tackle her. If she has a toy he wants to play with, he'll tackle her. This past weekend, she was standing at the fireplace and was playing with a lego. Apparently he wanted the Lego, because the next thing we knew he walked up behind her and slammed her into the fireplace, causing the side of her eyebrow to start gushing blood. After a few minutes of sobbing, Karlee was okay and ready to smile for the camera.
Then yesterday when we went to preschool to pick them up I noticed a nice round bruise forming on Karlee's cheek. The teacher informed us that Gunner got his first introduction to time-out because he tackled Karlee to the ground and held her down while he pinched her cheek. NOT COOL! These pictures are from last night when we got home. The bruise is actually much worse this morning.
We are at our wits-end with this little boy and don't know what to do and how to teach him to not be SO violent. I shudder to think what will happen if he does this to another kid...can you imagine if he gets expelled at age 1???
I'm open to suggestions, because so far butt-spankings aren't remotely working, and time-out's at school don't phase him. HELP!!!