House Intelligence Committee Chairman Adam Schiff (D-Calif.) spoke to reporters on Monday after four White House staffers subpoenaed to testify in the impeachment inquiry defied their orders to appear.

This also comes after the transcripts of two depositions – former U.S. ambassador to Ukraine Marie Yovanovitch and former former senior Mike Pompeo adviser P. Michael McKinley – were released to the public.

The subpoenaed White house officials included Senior aide to White House Chief of Staff Mick Mulvaney Robert Blair, White House director of natural resources, energy and science Brian McCormack, and National Security Council lawyers John Eisenberg and Michael Ellis.

Schiff cited the impeachment of President Nixon, and said that the four new refusals to appear could each contribute to charges of obstructing the investigation.