Police: Security officer's gunshot warning caused stampede

A stampede at a Fourth of July fireworks display that injured more than a dozen people at Chicago's Navy Pier started when a private security officer shouted for bystanders to take cover, police said Friday.Chicago police spokeswoman Kellie Bartoli said "approximately 13" people who fell or were trampled during the ruckus were hospitalized with minor injuries.The incidents happened just after 10 p.m. as thousands crowded Navy Pier on Lake Michigan's shoreline.