(CNN) At least 18 law enforcement officers fired their guns during last week's deadly shootout involving a hijacked UPS truck at a busy South Florida intersection, a police union official said Tuesday.

Most of those officers -- 13 -- were from Miami-Dade Police Department, having followed the truck into Broward County during a long chase, said Rod Skirvin, president of the Broward County Police Benevolent Association.

Four people were killed in the shootout Thursday evening at an intersection in Miramar: two robbery suspects who'd hijacked the truck; a UPS driver they had taken hostage; and a bystander, police said.

The shootout happened as a police chase ended after the hijacked UPS truck got stuck in traffic. Gunfire erupted from inside and outside the truck, though it's not clear who fired first.

Police officers left their vehicles and crouched behind cars -- their own and others at the intersection -- as shields as they approached, video from the scene showed