Oh, so let me get this straight.
1. Kaspersky finds out they were hacked
2. Kaspersky and FSB say Americans were behind the hack
3. Kaspersky and FSB refuse to provide any evidence for their claim
Yep! Totally not suspicious. If you were robbed and went to the police to report the crime, I bet you'd also refuse to give any details of the robbery, right?
I don't know if any of you here follow(ed) war in Ukraine, but in short, every time Russians accused someone of something, or bragged about something, they never provide the evidence for their claim. Never.