As investigators continue to look into what caused the fatal collapse of the Interstate 35 bridge in Minneapolis earlier this month, several suspects have come to the forefront.
According to Reuters, investigators are looking into whether de-icing agents used on the bridge over the years could have had a corrosive effect on the structure.
The National Transportation Safety Board says it is looking into the possibility that the bridge design and construction itself may have played a role in the collapse.
Investigators have also examined steel plates that helped connect support beams underneath the bridge decking.
But perhaps the most unusual suspect in this case is not a human one. The Associated Press reports that investigators are working to determine whether a buildup of pigeon dung on the bridge could have corroded key parts of the structure.
Pigeon dung contains ammonia and acids, which, over time, can dissolve even concrete.