[Gritty]

It would be a major job to place enough explosive to part the main suspension cables of the Mackinac bridge

Rather than bring it down, it would be a lot easier to plant several IEDs along the parade route and set them off simultaneously. I'm sure the terrorists have IEDs down pat by now, including their camoflage or even by using car bombs in the opposite lanes.

The casualties would be terrible in such a huge crowd, and the press would all be there to witness it, not to mention the Governor and numerous politicians. The carnage could be horrific, not to mention the added horror of those blown over the edge for the long, long trip down into the cold Straights,... you know, families and strollers and all that.

[Master of Orion]

Rather than bring it down, it would be a lot easier to plant several IEDs along the parade route

Nor discount the tried and true method of using suicide bombers.

[tommix2]

it would be a lot easier to plant several IEDs along the parade route and set them off simultaneously. Yes I think your are right. These knuckleheads aren't that stupid to think they could take the bridgew down. They probably had something like you say in mind.