The UFC has launched an investigation into legendary fighter BJ Penn -- after a former writer on Penn's website claimed BJ sexually assaulted his girlfriend.
The writer is Pedro Alex Carrasco -- who went to Twitter and said BJ recently came to his home and "when I wasn't there and forced himself on my girl, pulled her clothes off, then blamed me, said I looked at his GF ass."
Pedro's girlfriend reportedly filed charged is Ohio -- and the UFC has already announced it's looking into the situation.
"UFC is aware of the recent allegations made against BJ Penn.