"Explosive materials" could be camping propane, spray deoderant, detergent, rubbing alcohol, bourbon, firecrackers, or any of dozens of household (or apartment items). Unless there are exact matches in materials and direct connection to the explosion event, it is all meaningless.
That is how they search and blow up stuff, and that is how the media reports it. Conclusive reports are issued much later after all political considerations are weighed.
You'll have to pick your own truth from the tidbits, because the details are only going to get more muddled going forward.
The caption says:
Norman Fire Department members clean off debris from an explosion in the South Oval on campus
But clearly they are washing off a bus! Thus all the "bus" stories have at least a grain of truth in them. Hard to figure why an explosion that barely scorched the concrete around the bench it supposedly occured on, could throw "debris" (don't ask what sort) very far.
This photo from KOCO TV Channel 5 in OK City: