Federal investigators believe a deadly blast Thursday along U.S. 26 on Mount Hood was caused by a "substantial" amount of material that target shooters use to make exploding targets.

The explosion killed one person and left a crater in the ground just 10 feet off the highway.

Clackamas County sheriff's officials will now try to determine if the blast was a homicide, suicide or accident.

The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives determined that the blast didn't violate federal codes controlling explosives and turned the investigation over to the Sheriff's Office.

Authorities haven't released the identity of the victim yet. Dr. Karen Gunson, Oregon State medical examiner, confirmed the victim died of "blast injuries and body fragmentation," but said she could not yet make public his name.

Meanwhile, FBI agents swarmed a Northeast Portland residence Thursday afternoon, but declined to explain how -- or whether -- the two events were connected.

About 6:30 a.m. Thursday, the U.S. Forest Service reported an explosion roughly six miles east of Welches, bringing in federal agents, the Metropolitan Explosive Disposal Unit and dozens of local police. U.S. 26 was closed in both directions for about 12 hours during the investigation.

Brian Bennett, an ATF spokesman in Seattle, declined to speculate how much of the explosive material was used to create such damage. The crater left was more than 10 feet wide and 2 feet deep.

"All I can say is there was a large blast -- very large," Bennett said. "But we still aren't sure how much or exactly what kind of materials were involved."

Several companies make binary explosive target kits, which are commonly available at gun stores. Generally, they require mixing together an oxidizer and fuel before they become volatile. The mixture is then placed in a can or a jar that will explode when hit by a bullet.

"It takes a bullet to set them off," Bennett said. "At this point, how this material was ignited is one of the unknowns."

The kits, widely available on the Internet, require no background checks or proof of identity to buy, have no storage requirements and aren't regulated as an explosive.

The blast occurred just off U.S. 26 near Milepost 48. When investigators converged on the scene, they found a white car parked along the highway and a gaping hole in the ground.

They didn't know if there were additional explosives on the scene and used an armored blast-resistant robot to explore the site. Bomb squad technicians detonated several smaller charges to open a car found along the road, and the robot opened its trunk. The robot removed a backpack, which subsequently was X-rayed. But no additional explosives were found.

Later Thursday, authorities cordoned off a two-story fourplex in the 8500 block of Northeast Halsey Street in Portland. Neighbors reported seeing investigators taking items out of one of the units, which had the same address as the car registration.

An FBI spokeswoman briefly addressed reporters gathered nearby, but would not confirm any link between the apartment and the explosion.

The front door of one unit was taped off with black-and-yellow police tape, as was the empty parking lot. One neighbor said police arrived at the building about 8 or 9 a.m.

Shortly after 6:30 p.m., several agents left the apartment with a young man. One of the investigators tore off the tape from the apartment entrance, but the parking lot remained cordoned off. It was unclear who the young man was or his connection -- if any -- with the investigation.

Reporter Rebecca Woolington contributed to this report

-- Rick Bella rbella@oregonian.com

503-294-5915; @southnewshound