Fact is we (officially) strongly suspect it was them. Even the 'lost control' theory suggests they're a danger because they're not keeping things out of the wrong hands.

And what if they come - they're just going to say 'nope not ours'. You don't imagine they would say 'oh shit we fucked up and forgot to lock the door where we keep this stuff, sorry world we're idiots'.

Expecting us to cooperate with Putin in an investigation is pushing it. We'd be just as likely to cooperate with Iran or N. Korea in some investigation.