Your right to know.
Interviews are your main way of getting at the truth, but they're not the only way.
In most cases, when representing your members, you have the right to any "necessary and relevant" information the employer has. You can request this information anytime in the grievance process, including the initial investigation. Make the request in writing, being as specific as you can, and giving a reasonable deadline.
Some of the materials stewards can request include:
The union is also usually required to provide information to management if asked to do so.