LAKE BUENA VISTA, Fla. -- Disney's monorail system was running again Monday afternoon for the first time since Sunday morning's deadly crash. It killed a young man, 21-year-old Austin Wuennenberg, who family members described as "a genius."

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"PERFECT SON": Aunt Talks About Monorail Victim
SLIDESHOW: Images Show Crash Scene, Victim
VIDEO REPORT: Unanswered Questions After Crash

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"Austin was truly a genius," said Penny Shanahan, the victim's aunt (watch full interview). "Everyone had such great high hopes and expectations. They knew he would meet them and exceed them."

Federal and local investigators are still working to find out what caused the deadly crash. Two Occupational Safety and Health Administration (OSHA) investigators cleared the track for travel Monday afternoon and Disney's monorails were reopened.

The agency said Monday it will investigate whether workplace safety rules were broken before one monorail train crashed into another, said Mike D'Aquino, an OSHA spokesman in Atlanta. If citations are issued, Disney World could be fined thousands of dollars. The Orange County Sheriff's Office in Orlando also is investigating the death.

In addition to OSHA and the sheriff's office, the National Transportation Safety Board is investigating the accident. According to a release issued Monday afternoon, "Investigator Ted Turpin from the NTSB Office of Railroad, Pipeline and Hazardous Materials Investigations is responding to the scene."

Also Monday, an autopsy report was released for the young driver killed in the crash as well as 911 calls made from the park (listen to calls).

OSHA cleared the track to reopen, but the investigation into whether any safety rules were broken could take as long as six months to finish. In 911 calls released Monday, it was clear other Disney workers didn't even know right away that someone was killed in the crash.

Close-up photos obtained by Eyewitness News (see photos) show the force with which one Disney World monorail train crashed into another train early Sunday morning. It killed 21-year-old train operator Austin Wuennenberg, but at first employees who called 911 didn't know how bad it was.

"There's an issue with the monorail. Something happened," the caller said. "We just heard a loud bang and my manager ran in and told me to call you."

"A monorail was waiting to come into the station ... or it did not leave the station, and the other one cleared through the station, and there was a head-on collision," another caller said.

One other employee was rushed to a local hospital and Wuennenberg was pronounced dead at the scene.

"I was shocked. I still felt the chills from that," park guest Nelson Santana told Eyewitness News.

Santana said he saw Wuennenberg driving one of the monorail trains less than 48 hours earlier, when Santana and his family attended a citizenship ceremony at Disney World on Friday morning.

"The same smile in that picture, he gave it to us and he waved," he said.

Disney confirms it reopened the monorail system Monday afternoon, but with a new set of safety procedures to help workers verify trains are on the right tracks.

Disney spokeswoman Zoraya Suarez said extra measures to verify that the track switches are operating properly have been put into place, although she refused to be more specific.

"All I can say is there are additional verifications," she said. "We've supplemented our safety procedures and protocol."

Each train in the monorail system is run manually by the operator and must keep roughly 300 yards apart from each other. If a train gets too close to another, the train will automatically shut down and the operator will be replaced at the next stop, said Joseph Humphrey, a 30-year-old former Walt Disney World monorail operator, who stopped working at the resort in 2004.

A train operator has an emergency stop button at hand which can halt a train, as well as a button that can override the computer and allow the train to continue.

"For this accident to have occurred, one of two things had to happen," Humphrey said. "Either the computers had to have catastropically just failed or someone had to have engaged an override button."

The medical examiner's office won't issue a final report on Wuennenberg's death until toxicology tests are finished. Toxicology tests are standard procedure in any unexpected death, said Sheri Blanton, a spokeswoman for the medical examiner's office in Orlando.

BRIEF HISTORY OF DISNEY'S MONORAIL

Walt Disney World's monorail system began operation in 1971. The fleet includes 12 trains and each train has six cars.

There are six stations along the nearly 15 miles of track. On an average day, it's estimated that the monorails carry about 150,000 Disney guests.

Sunday's crash was the first fatal accident involving the system in 38 years. However, there have been at least three other troubling incidents.

In 1985, fire burned through a car on one of the trains. In 1991, two Disney employees were injured when one of the trains collided with a camera rig while a Disney crew was taping a commercial. In 1996, two drivers were treated for smoke inhalation after an electrical fire. All of the passengers were safely evacuated.