His death, under suspicious circumstances, objectively benefits Boeing in an ongoing criminal investigation.
This is motive, not evidence.
That seems like sufficient justification to conduct an investigation.
The fact that he has died is sufficient justification to conduct an investigation, and I’m sure they will. But the claim was that they have enough evidence against Boeing to subpoena basically everything they have. And Boeing having a motive to kill someone is not evidence that they did it, and would not pass a judge if anyone were to seek some kind of warrant.