They don't. That's why they did not say it wasn't, and they did not say it was. They said "at this time it doesn't appear to be so." On what basis might they say as much (or as little)? I can think of at least three very likely reasons: 1.) The bridge is old. 2.) There has been ongoing construction in the area with stress placed on the bridge in unusual places, and 3.) A singular Minneapolis bridge is not exactly a spectacular, let alone strategic, target.
Gut feelings.
Sleep tight.