And officials were contending with what they described as “sheer chaos” on Canfield Drive, where bystanders, including at least one of Mr. Brown’s relatives, frequently stepped inside the yellow tape, hindering investigators. Gunshots were heard at the scene, further disrupting the officers’ work.

“Usually they go straight to their jobs,” Officer Brian Schellman, a county police spokesman, said of the detectives who process crime scenes for evidence. “They couldn’t do that right away because there weren’t enough police there to quiet the situation.”

For part of the time, Mr. Brown’s body lay in the open, allowing people to record it on their cellphones. A white sheet was draped over Mr. Brown’s body, but his feet remained exposed and blood could still be seen. The police later shielded the body with a low, six-panel orange partition typically used for car crashes.

Experts in policing said there was no standard for how long a body should remain at a scene, but they expressed surprise at how Mr. Brown’s body had been allowed to remain in public view.