At least six people were killed and dozens seriously injured on Monday when a man opened fire from a rooftop on an Independence Day parade in Highland Park, Illinois.

After an hours-long manhunt, police arrested a 22-year-old man identified as Robert E. "Bobby" Crimo III. They did not give a possible motive.

On the roof from which the shooter fired, police said they found a "high-powered rifle" and a ladder attached to the building.

"This appears to be completely random," said Lake County Sheriff Sgt. Christopher Covelli. "Very random, very intentional, very sad day."