This was no accident. It was not only forseeable, it was forseen. BP has already released memos demonstrating their negligence and lack of concern for safety.
Prosecutors may not be able to prove deliberate criminal behavior, but the law makes it clear that BP is responsible for the damage they have caused. As it has been pointed out many many times, intent has nothing to do with laws where strict liability applies, and in this case, strict liability does apply.
Furthermore, when it comes to things like reasonable suspicion and probable cause, there is no doubt that such clear proof of criminal behavior (and poisoning those pelicans are a crime) allows the govt to detain, search and otherwise investigate a subject. And in such circumstances, the legal authorities do not delay the investigation.
Only the rightwing supporters of corporate welfare believe the investigation should be delayed.