Did Thick Smoke from a Fireworks Exhibit Cause Major Wreck on the UK M5?

At first, drivers thought it was fog. But it may turn out to be smoke. See the article here:

http://www.telegraph.co.uk/motoring/news/8873311/Rugby-club-fireworks-show-under-investigation-over-M5-crash.html
Seven people were killed and over 50 were injured in the resulting pile-up.