Witnesses told police a gas canister exploded to start the fire in a multilevel building which houses the Ycua Bolanos supermarket, a food court and a parking garage, causing the bottom floor to collapse.
The witnesses know what happened
This statement fits in the neither category, because we do not know if the the witnesses actually know what happened or are just guessing or making things up. The system may have assumed that because they gave a description of what happened, they definitely know what happened. 
( z1 know :ARG0 ( z2 person :ARG0-of ( z3 witness ) ) :ARG1 ( z4 event ) )