US officials have cited presidential privilege in withholding details of 24 emails related to President Donald Trump’s hold on military assistance to Ukraine, according to report on Saturday.

The revelation in a court filing around midnight on Friday came just hours after US senators holding Trump’s impeachment trial voted largely on party lines against hearing witnesses that could have shed further light on the hold, an issue at the heart of Trump’s impeachment for abuse of power and obstruction of Congress.

The White House’s Office of Management and Budget (OMB) “has redacted portions of 24 documents pursuant to the Presidential Communications Privilege,” OMB’s deputy general counsel Heather Walsh said in a court filing, cited by US media.

The filing is in response to a freedom of information request for documents related to the Ukraine aid.

The 100 members of the Senate have been sitting silently for six days listening to impassioned arguments for and against Trump’s removal from office for abuse of power and obstruction of Congress.

Democrats have been pressing for the Senate to subpoena Bolton, who was fired by Trump in September, after reports that his upcoming White House tell-all book corroborates the abuse-of-power impeachment charge against the president over his dealings with Ukraine.

On January 7, Bolton had said that he is willing to testify in the expected Senate impeachment trial of the president, a surprise development that could complicate a weeks-long dispute over how the trial would play out.

Earlier, Bolton was so alarmed by a White House–linked effort to pressure Ukraine to investigate Democrats, he told aide Fiona Hill to alert the National Security Council’s chief lawyer.

Ukraine’s former president had said that he discussed investments with President Donald Trump’s attorney, Rudy Giuliani, in 2017, but that he never discussed Ukrainian companies with any US official.

Democrats said he conditioned the aid on Ukraine’s announcing investigations, including into his potential 2020 election challenger Joe Biden.

“Specifically, the documents in this category are emails that reflect communications by either the President, the Vice President, or the President’s immediate advisors regarding Presidential decision-making about the scope, duration, and purpose of the hold on military assistance to Ukraine,” Walsh wrote.

She added that disclosure of documents identified as subject to presidential communications privilege “in this case would risk harming the quality of the information and advice available to the president.”

“Make no mistake, the full truth will eventually come out and Republicans will have to answer for why they were so determined to enable the president to hide it.”

The non-partisan Government Accountability Office ruled that OMB violated the law by withholding Ukraine assistance.

Bolton is among the witnesses the Democrats wanted to testify at Trump’s Senate trial.

On December 18, President Trump was formally impeached in a historic vote in the House of Representatives.

In September, the impeachment inquiry, which Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.

Democrats are hoping their arguments will at least persuade some Republicans, who hold a 53-47 majority in the Senate, to support their call to issue subpoenas next week for four top current and former Trump aides to testify, and for internal White House records about the Ukraine affair.

(With inputs from agency)