PORTLAND - Investigators on Thursday combed through the wreckage of a midair collision between two fighter jets over the Columbia River, but said they might not determine a cause for months.

Wednesday's crash, which killed a Marine reserve pilot and his observer and left the pilot of the other jet with minor injuries, is the latest in a string of U.S. Marine Corps aviation accidents, coming less than a month after two fatal accidents involving Marine fighter jets based in Beaufort, S.C.

An investigation team led by the Marine Corps set up camp Thursday afternoon on dry grassland overlooking the river gorge, near the northeastern town of Arlington, where the F/A-18 Hornets collided. Most of the debris fell in the river, investigators said.

Marines have been killed or injured in at least 10 aviation accidents since July 2003, according to news reports. Six of those involved F-18s, a fighter and ground attack plane used by both the Marines and Navy.

According to statistics compiled by the Naval Safety Center and reviewed by The State newspaper in Columbia, S.C., the Marines' accident rate involving a death or $1 million in damages could be the service's highest since 1990, if the pace of 4.9 mishaps per 100,000 flight hours continues through September.

But Capt. Jerome Bryant, a Marine Corps spokesman at the Pentagon, said a review of Marine aviation records from 1994 to 2004 for such mishaps did not reveal a pattern.

"There's no indication over a 10-year period of a trend," Bryant said. "Sometimes it's up, sometimes it's down."

On Thursday, about 30 Marine reservists and 50 volunteers from the Oregon National Guard searched the fields outside Arlington, tagging and mapping debris for investigators to find later, Guard spokesman Maj. Mike Allegre said.

Divers from the U.S. Air Force reserve searched the river, where 80 percent of the wreckage fell, he said. Meanwhile, 20 members of the Portland Air National Guard set up tents for the searchers, with power generators, air conditioning and lights.

Authorities say the fighter jets were returning from a low-altitude training mission Wednesday from the Oregon National Guard's air base in Portland to a bombing range in Boardman.

The planes had completed a flight to practice basic training maneuvers and were returning to base in what is known as "administrative flight mode," said Maj. Tom Nelson, a Marine Corps spokesman in New Orleans. That means the jets were likely flying level and in formation about 50 to 60 feet apart, he said.

Mechanical problems were likely not the cause of Wednesday's crash, according to Jaime Hunter, editor of Jane's Aircraft Upgrades.

"It must be something to do with operational reasons," Hunter said, calling the F-18 a "reliable and safe" airplane.

The squadron, based at Marine Corps Air Station Miramar in San Diego, arrived in Portland on July 11 for a two-week training mission. The training had included mock dog fights with "enemy" aircraft and attacks on surface targets, along with air-to-ground support missions.

Most pilots in the squadron had served seven to 10 years in active duty before joining the reserves, and fly regularly to maintain their skills, Nelson said. The unit had not been mobilized for duty in Iraq or Afghanistan.

Allegre, the Oregon Guard spokesman, said military air squadrons are frequent visitors in Oregon, where they use the bombing range east of the Cascade Mountains. The visiting units also practice dog fights with Oregon Guard F-15 planes, to test the airplanes' differing qualities in mock air-to-air battles. "It's a brotherhood and you take care of each other," he said.

The names of the three reservists involved in Wednesday's collision had not been released as of 11 p.m. Thursday.

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Associated Press Writer Andrew Kramer contributed to this report.