An investigation is underway to determine what caused a monorail at Walt Disney World in Florida to slam into another, killing one train’s driver.

The accident happened Sunday at about 2 a.m.

The crash claimed the life of 21-year-old employee Austin Wuennenberg. Six guests were treated on the scene for minor injuries. The driver of the other monorail was also taken to the hospital with minor injuries.

One early theory among people involved with Disney’s monorail system is that one of the trains was switching tracks, which requires the monorail pilot to override the anti-crash system, according to the Orlando Sentinel.

The monorails have data recorders which may provide more information on the cause of the crash.

Wuennenberg was a student at nearby Stetson University. One of his former classmates is quoted as saying the young man scheduled his classes around his Disney job and took precautions if he was working a night shift.

Brandon Ross says his friend would make the hour-long drive to the Orlando area the day before his shift so he wouldn’t be tired when he operated the train.

In an online profile, Wuennenberg referred to working 12- and 14-hour shifts recently.

Tags: Disney World monorail crash, monorail death, monorail investigation