Federal investigators on Sunday closely examined a stretch of rural Arizona highway near Hoover Dam looking for clues to the cause of a tour bus crash that killed seven Chinese tourists.

The six investigative team members would be measuring and photographing the site, evaluating the condition of the highway, and looking for skid marks and other clues, National Transportation Safety Board spokesman Peter Knudson said.

Click here for photos. WARNING: GRAPHIC IMAGES.

"These markings can be very important in telling the story of what happened," Knudson said.

The bus crashed Friday on a straight stretch of U.S. 93, about 70 miles southeast of Las Vegas.

The tourists had left Las Vegas early Friday on a trip to the Grand Canyon, and were returning when the bus veered right and then left across the median, rolling at least once before resting across the southbound lanes of the highway.

Along with the seven people killed, 10 others were injured.

After inspecting the crash site, the investigators planned to head south to Kingman to evaluate the bus. That will include checking whether the wheels and brakes were in good working order and whether any mechanical malfunctions may have caused the accident, Knudson said.

He said the investigators also plan to interview the 48-year-old bus driver, who was in fair condition at a Las Vegas hospital, and the surviving passengers.

"The more people we talk with, the more information we'll be able to get," he said.

The investigation will take 12 to 18 months to complete, with the bulk of the work being conducted at the NTSB's headquarters in Washington, Knudson said.

Representatives of the Arizona Department of Public Safety said they likely will have some preliminary results this week.

"Was it mechanical failure? Was it driver error?" DPS spokesman Lt. James Warriner said. "All that will come with looking at the vehicle and conducting interviews."

Warriner said of the weather at the time of the wreck that it was a "nice, clear day."

The DPS said the bus belonged to D.W. Tours of San Gabriel, Calif., which didn't respond to an e-mail from The Associated Press seeking comment.

Five people remained hospitalized Sunday at University Medical Center in Las Vegas. Two were in critical condition.

Hospital spokesman Rick Plummer said the victims' injuries ranged from spinal and head injuries to bone fractures. "It ran the whole gamut of injuries," he said.

Volunteers from the Chinese community in Las Vegas crowded the hospital's trauma unit Saturday, hoping to help with translation and taking food to the families of the injured.

Huang Xiaojian from the Chinese consulate in Los Angeles also was at the hospital but said she would not discuss details. "I am here to visit the patients," she said.

Two others — an 18-year-old woman and a 57-year-old man in fair condition Sunday — were at Sunrise Hospital in Las Vegas, according to hospital spokeswoman Ashlee Seymour. Another was still being treated at Kingman Regional Medical Center, hospital spokesman Ryan Kennedy said.