A "person of interest" has been arrested after a mass shooting at a July 4 parade in Highland Park in the U.S. state of Illinois, which left six people dead and more than 35 people injured, local police said on Monday.

The arrest follows an hours-long manhunt. Police are still describing the 22-year-old man in custody, Robert E Crimo III, as a "person of interest" while they investigate and connect him to the scene.