House of Representatives investigators are looking into whether President Donald Trump lied to special counsel Robert Mueller during the Russia probe.
"Did the president lie?" a House lawyer said in court as he requested files from the special counsel's inquiry.
Congressional impeachment lawmakers are reportedly scrutinising the president's statements to Mr Mueller on WikiLeaks.
The main thrust of the Democratic-led inquiry is on alleged abuse of power by Mr Trump in US-Ukraine relations.
Lawmakers first requested classified grand jury materials from the report in July.
A judge in October granted this access, though the Department of Justice appealed. Monday's hearing was to determine whether the appeal should temporarily block the previous grant of access.
Congress has seen most of the Mueller report - including some redacted parts - but certain grand jury material has remained secret.
Doug Letter, representing the House Judiciary Committee - which would eventually be responsible for filing articles of impeachment against the president - spoke to the federal appeals court on Monday.
He said the impeachment inquiry was looking into the Ukraine matter but revealed lawmakers were also investigating whether Mr Trump lied to Mr Mueller during the course of the probe into Russian meddling in the 2016 election.
Mr Letter has requested the redacted parts of the Mueller report and the full transcripts related to those portions.
Mr Trump did not testify in the Mueller investigation, but he did submit written responses to some questions from the Mueller team.
During Mr Mueller's testimony before Congress in July, he was asked whether Mr Trump's incomplete written responses showed he was not always telling the truth. Mr Mueller replied: "I would say, generally."
It is unclear what exactly Mr Mueller meant by his response, but Mr Letter told the court on Monday he believed it meant the president "had been untruthful in some of his answers".
US media report Mr Letter may have been referring to whether Mr Trump lied about contacts with WikiLeaks and knowledge that they were going to publish hacked Democratic emails.
Mr Letter added that evidence from the recent trial of Trump adviser Roger Stone strengthened lawmakers' argument for obtaining the secret material.
Stone, who was convicted last week of lying to Congress about his work with WikiLeaks, had several calls with Mr Trump during the 2016 election.
The president has said he did not know of any contact between his campaign and WikiLeaks and that he did not discuss WikiLeaks with Stone.
A lawyer for the justice department argued that the impeachment inquiry was not a judicial proceeding and thus it was not legal for lawmakers to view classified grand jury material.
But the precedent exists: during the impeachment proceedings against President Richard Nixon, a federal judge granted the House access to grand jury materials.
Following Monday's oral arguments, the appeals panel is considering the matter.
Ahead of the news of this additional thread in the inquiry, Mr Trump tweeted that he would "strongly consider" testifying to the impeachment inquiry "in order to get Congress focused again".
As he did with the Mueller report, the president has dismissed the impeachment probe as a "witch hunt".
....that I testify about the phony Impeachment Witch Hunt. She also said I could do it in writing. Even though I did nothing wrong, and don’t like giving credibility to this No Due Process Hoax, I like the idea & will, in order to get Congress focused again, strongly consider it!— Donald J. Trump (@realDonaldTrump) November 18, 2019
This week, eight individuals are due to testify before House lawmakers as a part of the impeachment proceedings.
On Tuesday, Lt Col Alexander Vindman, a Ukraine expert on the National Security Council who listened in on Mr Trump's July call with Ukraine's President Zelensky, will testify.
The July call sparked a whistleblower complaint that eventually led to the start of the impeachment inquiry.