The regular buzz of helicopter rotors was absent from remote canyon areas along the U.S.-Mexico border as California National Guard choppers deployed to support border enforcement remained grounded following a crash that injured nine.

A team from the U.S. Army Aviation Center at Fort Rucker, Ala., was set to arrive in San Diego on Thursday to begin a federal investigation into the cause of Tuesday's accident in the mountains east of San Diego.

Officials have not commented on the cause of the crash of the 1973 UH-1 Huey transport helicopter, which went down while transporting a Border Patrol team to inaccessible areas for operations against human smugglers.

Two National Guard soldiers and three Border Patrol agents remained hospitalized with neck and back injuries, said Col. Kevin Ellsworth, commander of the joint mission along the California border. In all there were five agents and four Guard members aboard.

Officials grounded the six remaining helicopters on border duty, including one Huey and five OH-58 observation craft. The Guard's six other Hueys elsewhere in the state remain operational.

"Any time there's an accident there's a safety stand-down," Ellsworth said. Any decision to put the helicopters back in the air will come from state investigators, he added.

The crash occurred about 3:40 p.m. on Otay Mountain about 20 miles southeast of downtown San Diego and a few miles north of the international border. The craft, which had picked up the agents at Brown Field, came to rest on its belly, its main rotor snapped off and tail broken.

Col. Mitchell Medigovich, an aviation expert who is leading the California National Guard's investigation, said the helicopter was one of six Vietnam-era Hueys still flown by the state Guard.

"We don't want to speculate that the cause of the accident was anything to do with the age of these helicopters," Medigovich said, noting that the helicopter had undergone regular maintenance.

Medigovich said the UH-1 pilot and co-pilot were both experienced fliers with years of experience.

Ellsworth said all crew members credited the pilots with bringing the helicopter down as gently as possible in rough terrain.

"The wreckage looks pretty bad," Ellsworth said. "It could have been devastating, and all the people aboard survived."

Officials would not release any details of the names, ages or ranks of those aboard, citing privacy concerns.