(CNN) Senate Minority Leader Chuck Schumer on Monday ramped up his push for the Senate to pursue witnesses and documents in the impeachment trial of President Donald Trump after newly released emails raised questions about how aid to Ukraine was formally frozen on the same day of Trump's July call with the Eastern European country's leader.

Schumer sent a letter to all senators on Monday detailing specific documents he wants produced for the Senate trial, where Schumer and Democrats are pushing for witnesses that Senate Majority Leader Mitch McConnell has resisted in the early negotiations over the parameters of the trial.

Schumer cited the new emails from White House budget official Michael Duffey, which a judge ordered released to the Center for Public Integrity following a Freedom of Information Act request. In an email sent roughly 90 minutes after Trump's July 25 call with Ukrainian President Volodymyr Zelensky, Duffey told the Pentagon not to disburse the security assistance to Ukraine and to keep the information "closely held" due to the "sensitive nature" of the directive.

An OMB spokeswoman said that tying the freeze in aid to Trump's July call was "reckless," noting that the Pentagon and other agencies had already been notified the aid would be held a week earlier.

In his letter, Schumer noted that the House heard from an extensive group of witnesses in the impeachment inquiry, but investigators could not get access to most documents they sought because the administration defied congressional subpoenas.

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