In a June 26th decision, the NLRB overturned a rule, in place since 1978, that held that witness statements did not need to be produced in attempts to furnish information to investigations. The 1978 rule has been determined to be “flawed”, and replaced with a “balance test” burden for employers. Employers would have to satisfactorily establish an existing need to prevent cover-ups and evidence fabrication in order to lawfully refuse the release of witness statements to unions investigating grievance processes.
See "NLRB Reverses Longstanding Rule: Employers Now Required to Disclose Confidential Witness Statements in Investigations – a Major Change for Labor Arbitration?", Steven M. Swirsky, July 2, 2015