Shots were fired just before 2 p.m. at the New River Community College campus at the mall, where two women sustained gunshot wounds.
MacInnis of Christiansburg was in custody, WDBJ-TV, Roanoke, reported. He completed the Christiansburg Citizen Police Academy in 2012, Christiansburg Police Chief Mark Sisson told the TV station.
"Today has been a very tough day," Sisson said.
Sergio Pollock said he saw a man with a gun walk into the lobby of the community college and point the gun at a woman. People inside the mall said they heard at least three gunshots, The Roanoke Times reported.
The condition of the shooting victims was unknown.