Illinois State Police said in a statement Aaron Jackson of Washington Park was arrested in the Relax Inn motel in Alorton at 3:55 p.m. by Alorton police. Jackson was picked up on an outstanding warrant after police received a tip from the public, state police said.
Jackson was being held on $1 million bond.
Thornton, 52, was found in his car, which had left a city street and crashed into a tree. Police said he had been shot twice in the chest at close range.
At the time, investigators said the mayor had been shot by someone who wanted to rob him.
2014: The Year in Music [PHOTOS]