More than a month after a school bus crashed at Denver International Airport killing the driver and seriously injuring three football coaches, police say there is no explanation as to why the accident happened.

The Denver Traffic Investigation team released its reports and the only video showing the crash. The police department is closing the case and said although rare, the cause is inconclusive.

"We simply are going to have to call this one a mystery,” said Sgt. Mike Farr.

The fatal school bus crash happened at Denver International Airport on Sept. 11 just after 4 p.m.

DISCLAIMER: The following video shows the bus crash from several feet away. It may be difficult for some to watch. 9NEWS is choosing to publish the video to get a better understanding of how and why the crash happened.

As cars drive to pick up passengers at DIA, the security camera video captures a yellow school bus crashing into a cement pillar. It is the only video police saw of the crash.

"Is there something that helps explain the story? And here, there really was not,” said Farr.

School bus driver Kari Chopper picked up Legacy High School students and coaches after a football trip to California. Students on the bus told police they were expecting to leave the airport grounds but instead Chopper returned to the terminal and didn't say why.

And we may never know.

"I just want to know what happened. I wanna know what happened. It's hard for me to not know,” said Chopper’s husband Joshua.

He spoke with 9NEWS shortly after the crash but his questions still remain.

"We really try to provide answers,” said Farr.

Farr said the investigation is inconclusive after determining the bus was in good working condition and an autopsy showed Chopper had no drugs or alcohol in her system. They didn't uncover any medical conditions and said there is no evidence she crashed the bus intentionally.

"Certainly a good portion of my job is to provide answers for loved ones for family members directly and indirectly affected by this and it's troublesome when you're unable to do that,” he said.

The coroner ruled the 43-year-old's death an accident and after an investigation, the case is now closed.

DPD looked into whether Kari Choppers' cell phone was used before the accident and they said there is no evidence she was on the phone.