"We understand each of the individuals are making their own decision," he added.

Cooper, a career official, gave a deposition under subpoena behind closed doors in October and is set to testify on Wednesday before the House Intelligence Committee in the public portion of the investigation into President Donald Trump's dealings with Ukraine.

"For you to send it, it does create a chilling effect," Hirono replied.

Norquist argued the Pentagon had attempted to "set the right tone" and "be very professional and factual" with its letter to Cooper.

"My only point is I would have felt it inappropriate to not have their lawyer be aware of this information, and so that's why we shared it," he said.

As with other agencies, the Pentagon has defied subpoenas issued by the three House committees overseeing the impeachment probe.

Norquist was also pressed by Sen. Tim Kaine (D-Va.) to ensure military personnel who cooperate with congressional investigations will be protected by the Defense Department.

Kaine cited reports that Army Lt. Col. Alexander Vindman and his family are being provided security and could be moved to a secure location on a military base. Vindman, who works on the NSC and testified publicly on Tuesday, has faced attacks from Trump and his supporters.

Norquist didn't discuss Vindman but said Pentagon leaders take cooperation with Congress and the security of its personnel "very seriously."

"I'm not going to comment on any measures we take [with] individuals on personal security but we do take it very seriously, and we expect people to be responsive and truthful in their dealings with Congress," he said.

"If we think there's a security issue, we...deal with it, we deal with local authorities," Norquist later told Hirono.

Kaine also urged Norquist to be "very diligent in protecting members of our military if they are cooperating with Congress."

"Your words delivered here should hopefully give some assurance and some confidence to some who are very very worried," the Virginia Democrat said. "And I know they're worried because their families are calling my office. They're my constituents and they're nervous about what might happen to them."