WASHINGTON (BLOOMBERG) - New documents show President Donald Trump asked about United States military aid to Ukraine a month before the call with Ukrainian President Volodymyr Zelensky that went on to trigger the impeachment investigation into Mr Trump.

The heavily redacted documents, obtained under a Freedom of Information request by the Centre for Public Integrity (CPI), also show that administration officials ordered a hold on the aid about an hour after the leaders' call on July 25.

Mr Trump asked about a June 19 article in the Washington Examiner newspaper referencing the aid. Mr Michael Duffey, an official at the Office of Management and Budget, wrote to the Pentagon requesting more details about the funding, according to an e-mail from Mr Duffey to the Pentagon comptroller. "The President has asked about this funding release, and I have been tasked to follow-up with someone over there to get more detail," said the e-mail.

An e-mail from Mr Duffey to Deputy Secretary of Defence David Norquist on July 25, shortly after the Trump-Zelensky phone call, expressed uneasiness about the hold.

"Given the sensitive nature of the request, I appreciate your keeping that information closely held to those who need to know," Mr Duffey wrote.

Mr Duffey, an OMB official working on national security issues, is one of four witnesses Senate Minority Leader Chuck Schumer has requested for the pending Senate trial of Mr Trump on two articles of impeachment passed by the House of Representative this week.

Mr Schumer met Senator Majority Leader Mitch McConnell last Thursday to discuss the terms of a trial. The meeting ended in an impasse before Congress broke for the holidays.

Mr Schumer "made clear to Senator McConnell that the witnesses and documents are necessary to ensure a fair trial in the Senate", spokesman Justin Goodman said. Mr McConnell has been pushing for a process that likely won't include any witnesses.

The other officials Mr Schumer has requested are Mr Mick Mulvaney, the acting White House chief of staff, Mr John Bolton, the former national security adviser who left the administration in September, and Mr Robert Blair, an adviser to Mr Mulvaney.

The House adopted two articles of impeachment against Mr Trump on Wednesday following an investigation into allegations that the President withheld military aid from Ukraine in an effort to extract politically damaging information about a domestic political rival, former vice-president Joe Biden.

The historic votes on Wednesday won the support of almost all Democrats in the House chamber but not a single Republican, leaving Mr Trump as only the third president in US history to be impeached.

Senator Chris Murphy, a Democrat from Connecticut, said on Saturday on Twitter that the new documents, which CPI - a non-profit investigative journalism organisation - reported late last Friday, show that the internal notes show Mr Trump's "direct involvement" in the matter.

Mr Murphy later seemed to concede that the documents, while important, were just the latest piece of evidence in a months-long inquiry spearheaded by House Democrats that still seems certain to end in Mr Trump's acquittal by the Republican-controlled Senate, some time in early 2020.

"Frankly, it's just the 77th piece of evidence confirming the same thing," Mr Murphy said in a reply to Senator Brian Schatz, a Hawaii Democrat.