I was working on a problem in the field engineering. I have used one approach and it turned out that it does not work well. After that I got into conflict with my supervisor, and realized that it will be hard to publish the original paper with him. So, I have redone everything with another approach, wrote a paper, submitted it and at as soon as my paper have been almost accepted, my supervisor wrote to the editor claiming that he also should be an author of the paper. He haven't even seen the new paper nor haven't analysed or interpreted the new results.
Now my supervisor has started a university investigation against me, with following allegation:
"Dr.X submitted a manuscript on [some topic] to the journal Y. Theory and simulations of [some topic] are part of his responsibilities in [grant] for which Prof. #1 and Prof. #2 are co-PIs. The manuscript was submitted as a sole author without the prior knowledge of either coPI. Dr. X refused to provide a copy of the manuscript. The work described in the manuscript is unlikely to be Dr. X's independent work given his background prior to joining the university and given his current work under [grant]."
How should I respond to the allegations of Prof. #1?