Atkinson determined earlier that the whistleblower complaint was an "urgent concern" that "appears credible", US-based Axios media reported Friday, citing unnamed sources familiar with the matter.
According to the media report, Atkinson is likely to give the most detailed testimony on the issues raised by the complaint, short of a committee hearing with the whistleblower.
House Democrats have launched an impeachment inquiry based on a whistleblower complaint that alleges presidential misconduct in a 25 July telephone conversation between Trump and Zelensky.
House Speaker Nancy Pelosi accused Trump of seeking Ukraine’s help to investigate Joe Biden’s son and of using defence assistance to Ukraine as a bargaining tool. Pelosi said she is directing six congressional committees to proceed with official impeachment inquiries into the matter.
Trump has denied the allegations, saying the Democrats are engaging in a "political witch hunt" to try to reverse the 2016 election.
The US president allowed the release of an unredacted transcript of his conversation with Ukrainian President Volodymyr Zelensky.
The transcript revealed that Trump asked Zelensky to work with his personal lawyer and the US attorney general to "look into" the case of Joe Biden, who admitted publicly in 2016 to having threatened Ukraine authorities with withholding $1 billion in US loan guarantees if Kiev did not fire the prosecutor who had been investigating a Ukrainian gas company linked to his son, Hunter.
The transcript did not, however, contain signs that the US president overtly threatened to withhold financial assistance to Ukraine if Zelensky did not do him this favor, but Democrats are accusing Trump of using his power to request evidence against a potential competitor in the next presidential race.
Earlier in the day, the US House of Representatives committees on Foreign Affairs, Intelligence and Oversight subpoenaed Secretary of State Mike Pompeo for documents related to the Trump-Zelensky impeachment inquiry. The chairmen stressed that failure or refusal to comply with the subpoena by Pompeo will be considered evidence of obstruction of the impeachment inquiry.
