MILLICAN, Ore. -

An explosion that killed two people inside a World War II-era tank destroyer occurred as a round was being fired from its gun, Deschutes County investigators confirmed late Thursday.

“The exact cause of the explosion is still under investigation,” Sheriff Shane Nelson said.

Meanwhile, the Oregon state medical examiner completed autopsies Thursday morning on the two Oregon City men killed in Tuesday’s blast, Steven Todd Preston, 51, and Austin Tyler Lee, 22. Nelson said the medical examiner had yet to complete the final report.

Sheriff's Search and Rescue crews conducted a grid search Thursday for more evidence at the site 24 miles east of Bend where the explosion occurred inside Preston's 1944 M-18 "Hellcat" tank destroyer.

"Searchers located several items that were logged as evidence," Nelson said, declining to reveal any details due to the ongoing investigation.

About 20 SAR members conducted the search "to make sure we don't leave behind any  evidence," Sgt. Nathan Garibay said earlier Thursday. He said it was a standard procedure, with "SAR teams trained to do evidence searches" for area police agencies.

Nelson also said the district attorney's office will review the case.

The explosion occurred shortly after 3 p.m. Tuesday near a public firing range off U.S. Highway 20E.

They are being assisted by the Oregon State Police Arson and Explosives Section, as well as the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. The investigators are working closely with the Deschutes County District Attorney’s Office.

ATF officials said Wednesday while you are allowed to have a tank in Oregon, they issue permits only for the gun on the tank and not the tank itself.

Autopsies are planned later this week at the Oregon State Medical Examiner’s Office near Portland.

Sheriff's deputies, OSP troopers and Bend Fire Department medics rushed to the scene when word came shortly after 3 p.m. Tuesday of the explosion, on BLM land, Garibay said.

"This is a tragic day," Nelson told NewsChannel 21 at the scene Tuesday night. "Two families lost loved ones, and our hearts and prayers go out to those that loved ones today."

Preston was the owner and president of Sergeants Towing Inc. in Portland. One employee said Preston was a wonderful man and they are just focused on keeping his company working. Preston leaves his wife and two daughters behind.

He was also the board director for the Military Vehicle Preservation Association. The organization's president, Kevin Lockwood, said Wednesday safety is a very high priority for them, but he added that any activity with machinery involves the danger of something going wrong.

Lockwood said he knew Preston for seven years and said he was very knowledgeable about the hobby he had been practicing for 15 years.

"I know that Steve had been around this particular type of heavy equipment for a long time and he has exercised good judgement and safe practices," Lockwood said.

An AirLink helicopter was called out to the scene, which Garibay said was located adjacent to the Central Oregon Shooting Sports Association range. But authorities said one man died in the blast, before first responders arrived, and the other could not be saved.

"One of the victims was still alive (when police arrived), and they were able to speak with that victim," Nelson said. "Bend Fire Department did attempt life-saving efforts on that victim, but the victim died."

The sheriff also said a film crew was present at the time of the explosion and was capturing video of the tank destroyer firing rounds, which could prove crucial as authorities work to learn what took place and why.

The video was to be used as part of a display of the tank destroyer in a future exhibit, Nelson said.

NewsChannel 21 did a story four years ago that showed the thank destroyer firing its weapon, with Discovery Channel on scene filming.

In Lone Pine in October 2011, after 12 years of restoration work, Preston brought his tank destroyer, emblazoned with the name "Rachel," to fellow military collector Chuck Hegele's 300-acre property in the Lone Pine area near Terrebonne to test fire the restored 76 mm gun, with a Discovery Chanel film crew on hand.

"I've owned (the tank destroyer) for about 12 years, but it was just recently that I made the gun live on it, so I actually have an operating cannon on it," Preston said at the time. "Chuck is an explosives expert friend of mine, so he's going to help me load it up."

The two men, whose love for military collectibles started at a young age, said they had filled out plenty of paperwork with the ATF to be able to do the tank firings. They said the hobby was relaxing, even therapeutic.

Preston said, "I think it's fun, just being able to load a live round into the chamber of that and pull the trigger."

Hegele, who puts on annual public cannon shoots twice a year, said, "How many countries can you run a live tank around, a full military tank? I'm privileged to live in a country like that."

Preston was heavily involved in the Military Vehicle Preservation Association, which posted a page on his passing. It said he was a board director and convention chairman, and asked all members "to keep Steve's wife, Rachel, and two daughters in their thoughts and prayers. We were all touched and enriched by Steve, and will miss our dear friend."

Dennis Ripp, who is president of the Military Vehicle Collectors Club of Oregon, told The Associated Press he has known Preston for at least 15 years.