For a few terrifying moments, environmentalist Rene Umberger found herself 50 feet underwater and unable to breathe.
Umberger, 53, was scuba diving off the coast of Kona, Hawaii, last Thursday, looking to document damage to a coral reef, when another diver rushed her and ripped away her breathing apparatus.
In a dramatic assault captured by two video cameras, Umberger manages to get her regulator back in place and breathe again, but was left shaken and angry.
"I was really afraid," she tells PEOPLE. "I was shocked. It happened so quickly, I just went into survival mode."
Umberger believes the attacker was somebody collecting reef fish for aquariums who didn't want to be filmed. Environmentalists and fish collectors have an ongoing dispute but it had never become this violent.
The State Division of Conservation and Resources Enforcement is investigating. Reef fish may legally be collected under certain rules, though environmental groups object to the practice and have been videotaping collectors to build a case against it.