Amazon, union tussle over identifying witnesses as U.S. labor board starts hearing lost a tussle on Friday over whether it could learn the names of witnesses before they testify at a hearing investigating claims that the company illegally interfered with a vote on unionizing an Alabama facility. warehouse workers outside Birmingham, Alabama, voted against forming a union by a more than 2-to-1 margin in early April, a major win for the retailer.

The Retail, Wholesale and Department Store Union asked the National Labor Relations Board to set aside the vote because Amazon allegedly interfered by threatening layoffs or closure of the facility if the union won.

Harry Johnson, a lawyer who spoke for Amazon at the first day of the hearing on Friday, said it would be a “trial by ambush” if his side did not know who would speak before each session.

“We wouldn’t be interested in finding out the email address and telephone number,” he said. “But the identity of the witness and what objections they would address … is actually some fundamental notice that we think should be given.”

Speaking for the union, Richard Rouco opposed giving a witness list in advance. “Protecting and guarding the identity of witnesses, employee witnesses in particular, until the moment that they’re prepared to testify is something that’s very important,” he said.

Hearing officer Kerstin Meyers denied Amazon’s request but indicated that she would accept a briefing on the matter. The hearing is expected to continue at least another week.