13Raw: Exclusive Video of Cedar Bridge Fire and Latest on Arson Investigation

WINTERSET, Iowa  --  Channel 13 News has obtained exclusive, never before seen video of the Cedar Bridge fully engulfed in flames.

The images were captured by a cell phone camera recorded by a man who lives near the bridge. He was on his way to work when he saw the fire and called 911.

The Madison County Sheriff's Office announced a third arrest in the arson investigation on Tuesday.

"We wanted to get everything in line, all our ducks in a row before, because these are serious, serious charges. I mean, you're gonna disrupt somebody's life at this point and you want to make sure you're right," said Sheriff Jason Barnes.

After putting the pieces of the puzzle together, Sheriff Barnes and the State Fire Marshal took a trip to South Carolina.

"She went to basic training the next day after the fire, the very next day she went down there, so about three days later we met her," said Sheriff Barnes.

On Wednesday, 19-year-old Alivia Marie Bergmann of Des Moines was arrested on charges of 2nd degree arson and 1st degree criminal mischief. Alex Hoff, 17, of West Des Moines and Joel Davis, 18, of Norwalk are also charged with arson.

"She knew those guys maybe a month, month and a half is what the investigation is saying. (They) weren't close at all, which makes it all the more why? You know?" said the sheriff.

Hoff was reportedly the ringleader.

"I just wish one of the three would have said, 'hey, I don't think this is such a good idea.' I mean, you just need one to say that. Unfortunately, I found no evidence that that happened."

Authorities believe they know not only how, but why the bridge was set on fire.

"There was a motive, there was a reason, that bridge was a target," said Sheriff Barnes. "There was experiences at that bridge that come into light, whether it be an ex-girlfriend or something like that, but yeah, we think we have the reason why. As silly as it is, that was the reason."