Senate Minority Leader Chuck Schumer called on Defense Secretary Mark Esper to “do more” to protect those who have complained about President Trump, including Lt. Col. Alexander Vindman and Deputy Assistant Defense Secretary Laura Cooper, who will testify publicly this week during impeachment hearings.

“Recently, in response to questions from the media, you stated that LTC Vindman should not fear retaliation and that the Pentagon has protections for whistleblowers,” Schumer, a New York Democrat, wrote to Esper on Monday. “While I welcome your statement, I believe the Department of Defense must do more to formally ensure that all Department military and civilian personnel understand that they may make protected disclosures to Congress free from retaliation.”

Schumer also demanded Esper “cease any efforts” to block Pentagon employees from “cooperating with Congress” in the ongoing impeachment investigation and turn over documents lawmakers have requested as part of the investigation.

Vindman testified in closed-door proceedings that he “did not think it was proper” for Trump to ask Ukraine to investigate the actions of former Vice President Joe Biden.

Trump wanted Ukraine to investigate Biden’s efforts to oust a prosecutor in Ukraine who was targeting a gas company that employed Biden’s son, Hunter Biden.

Schumer said he was concerned about the safety of those testifying against Trump and pointed to Trump’s tweets criticizing former Ambassador Marie Yovanovitch during her testimony on Thursday.

“I request that you formally notify all civilian and military personnel of their legal rights to make protected disclosures to Congress,” Schumer wrote to Esper. “I also request that you brief me on what actions are being taken to ensure that LTC Vindman, Ms. Cooper, and other whistleblowers like them are afforded appropriate protections—both from workplace reprisals and for their personal safety and that of their families."