The gunman's father was later found shot to death in a rural area 50 miles away.
Investigators had no motive for the shootings.
Coos County District Attorney Paul Frasier said authorities believe Zachary Levi Brimhall, 34, of Dillard shot and killed David Jesse Hortman, 43, of Walker, Michigan, at the beach before killing himself.
Frasier, however, did not immediately blame Brimhall for the death of his father, William Ray Brimhall, 58, of Dillard, who was shot multiple times near his vehicle on a remote logging road in the Coast Range east of Coquille.
Investigators found several firearms, some large firecrackers used to frighten seals, and materials to make an improvised explosive device in Zachary Brimhall's vehicle and called the Oregon State Police bomb squad, Frasier said. Bomb technicians found no explosives in the father's vehicle.
Authorities said Hortman represented a recreational vehicle supply company and had decided to stay for a week of vacation after exhibiting at a trade show.
Mark MacPherson, whose car was also shot, told KCBY he thought someone was throwing rocks at his vehicle, "but my window shattered and I knew in that instant someone was shooting at me."
There was no known connection between Hortman and Zachary Brimhall, authorities said.
The shooting occurred early Tuesday at Bastendorff Beach just south of Coos Bay. The area has long been a popular spot for homeless people as well as people who want to have a campfire on the beach.
About a year ago, the federal Bureau of Land Management cut maximum camping time from 14 days to 24 hours at the site and stepped up patrols due to problems.
"We have had some problems in the past with some nefarious activity out there," agency spokeswoman Megan Harper said. "We have tried to step up patrols lately and it had seemed like it was getting a lot better.'
Dillard Store & Deli owner Keith Sjogren said Ray Brimhall had worked at a wood products mill and lived near the store, though Sjogren had not seen him for about six months. Sjogren described him as an easy-going man.