The Office of Management and Budget released a trove of documents related to Ukraine military aid to American Oversight on Tuesday following an FOIA request.

Why it matters: Allegations that Trump froze nearly $400 million of congressionally approved military aid in order to pressure Ukraine to investigate a potential 2020 rival are central to the case against the president in the Senate impeachment trial. Trump denies any wrongdoing over the aid.

Details: The documents were released just before midnight, two minutes before the deadline, after American Oversight made a "request for directives and communications that may relate to any effort to pressure the Ukrainian government to investigate one of President Trump's political opponents as part of an effort to give the president an electoral advantage," the nonpartisan oversight watchdog said.

The documents show email exchanges in late June following the publication of a Washington Examiner article on military assistance for Ukraine.

in late June following the publication of a Washington Examiner article on military assistance for Ukraine. The email exchanges include ones between acting assistant secretary of defense for international security affairs, Michael Duffey, the OMB's associate director for national security programs, and Mark Sandy, an OMB career official.

include ones between acting assistant secretary of defense for international security affairs, Michael Duffey, the OMB's associate director for national security programs, and Mark Sandy, an OMB career official. Duffey, Sandy and other officials appear to be hammering out details on the Ukraine Security Assistance Initiative (USAI) in the emails — dated from early August to Sept. 30.

appear to be hammering out details on the Ukraine Security Assistance Initiative (USAI) in the emails — dated from early August to Sept. 30. USAI refers to a defense program in which the Pentagon allocates money to Ukraine.

in which the Pentagon allocates money to Ukraine. "Additional emails through the summer and early fall, when Pentagon official Elaine McCusker was raising concerns about the legality of the freeze, are heavily redacted," American Oversight noted.

Flashback: Sandy testified that he was told in a July 12 email that Trump "is directing a hold on military support funding for Ukraine," but that he was not given a reason for the hold until early September.

Duffey was one of the top four political appointees helping White House acting Chief of Staff Mick Mulvaney withhold aid to Ukraine, per the New York Times. On Tuesday night, House Democrat impeachment managers tried to subpoena Duffey to testify in the Senate trial.

What they're saying: American Oversight's executive director Austin Evers said in a statement, "President Trump's lawyers stood in the Senate on Tuesday arguing that documents are totally unnecessary for the impeachment trial, but these documents give lie to that entire position."

"Despite the Trump Administration's obstruction and the rhetoric at the trial, the public can now see even more evidence of the president's corrupt scheme as it unfolded in real time. The volume of material released, and the volume of material still secreted away, only highlights how much the administration has withheld from the House, the Senate, and the American public."

— Austin Evers

Axios has contacted the White House and the OMB for comment.

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Editor's note: This article has been corrected to remove Katie Wheelbarger's name. The Defense Department official was not among those in the email exchange included in documents released to American Oversight.