Authorities are investigating an area near the Backwater Bridge in North Dakota where law enforcement reported hearing an explosion around 3 a.m. on Nov. 21.

According to the North Dakota Highway Patrol, several improvised weapons including small propane tanks rigged as explosives, as well as various slingshots and rocks, were found at the site.

The NDHP says the improvised weapons were used during a riot at the Backwater Bridge on Nov. 20 in attempts to harm law enforcement.

Authorities are continuing to investigate the area.