I don't know the exact answer, but one way would be one the legal premise of constructive possession. If it is within reach, and you had knowledge of it, then constructive possession says it is yours. This is generally used when no one wants to own up to it and they usually don't end up with a successful prosecution.

By the way, the info that they tested negative came from THEIR attorney and the results are confidential so no one is going to refute them at this point so take it for what it is worth.