Those who saw a helicopter, flashing emergency lights, and a throng of bleeding people at the University of Minnesota today can rest assured: It was just a drill.

Along the U of M Transitway, a bus was tipped on its side, simulating a light-rail train that had derailed after a bomb went off. Law enforcement and university staff were testing their communications systems and emergency response.

The Green Line, which will connect the downtowns of St. Paul and Minneapolis, opens June 14.