Three people were killed in a shooting at a park in the US state of Iowa and the suspected gunman is also dead, police said in a statement.

Police were notified of a triple homicide at the Maquoketa Caves State Park campground in Jackson County at about 6.23 a.m. on Friday, Xinhua news agency quoted the statement as saying.

Officers located three deceased bodies and later found that a camper was unaccounted for, it said.

Officers also found the deceased body of Anthony Orlando Sherwin, 23, west of the park.

Sherwin appeared to have suffered a self-inflicted gunshot wound.

Police said the Maquoketa Caves State Park, which stands northwest of the city of Maquoketa, is closed until further notice but there is no danger to the public.

Iowa Governor Kim Reynolds said in a statement that she's "horrified by the shooting this morning at Maquoketa Caves State Park and devastated by the loss of three innocent lives".

Gun violence has taken more than 24,000 lives across the US so far this year, according to the latest data from Gun Violence Archive.