BBC NEWS | Americas | Six dead as US bridge collapsesThe initial reports said "not a terrorist event", but of course that's unknowable. Later reports used "No indications", which is at least plausible.
...Russ Knocke, a spokesman for the Department of Homeland Security, told the AFP news agency: 'There are no indications of a nexus to terrorism at this time.'
We're a pretty wealthy state without any recent history of major infrastructure problems, and there's been no significant work on that bridge recently. Bridge engineering is supposed to be very well understood. So either our engineering textbooks need a rewrite, or there was something wrong about the initial construction or later repair of this particular bridge (and its peers?), or somebody blew it up. The last seems very unlikely but the other explanations aren't too likely either.