According to the Baltimore Sun, the man arrested at the scene was 24 year old Mujtaba Rabbani Jabbar of Owings Mills.
In what police described as a "bizarre" and "random attack", the Sun gives this account:
About 8:20 p.m., officers from the Franklin precinct were dispatched to a reported shooting at the Valley Center 9 cinema in the 9600 block of Reisterstown Road during the 8 p.m. showing of X-Men: The Last Stand. Police said they found the victim slumped over in a seat four rows from the back.I guess we'll have to wait to find out which mosque he attended.
According to authorities, immediately after the shooting, Jabbar walked up to a counter in the lobby where the theater manager was seated, placed the gun on the counter and said, "I just shot someone."
The manager called police, and the man waited for officers to arrive and arrest him, authorities said.
Update: Corrected spelling of suspect's name.