"We will move as expeditiously as possible [...] But we have to see what witnesses are going to make themselves available and what witnesses are going to require compulsion", House Intelligence Committee Chairman Adam Schiff said, cited by CNN.
Earlier in the day, Trump called on Schiff to resign and be investigated over what the beleaguered US president called a fictitious account of his phone conversation with Ukrainian President Volodymyr Zelensky.
During a House hearing on Thursday, Schiff provided an interpretation of the 25 July Trump-Zelensky telephone conversation. The lawmaker later said his reading of the transcript was intended to be partly a "parody".
On Tuesday, the Democrat-controlled House of Representatives launched a long-awaited impeachment inquiry over allegations that Trump, in an attempt to boost his 2020 re-election bid, pressed Zelensky to probe the business activities of former US Vice President Joe Biden’s's son in Ukraine in the July phone talk. House Dems allege that Trump threatened to withhold military aid to Ukraine unless Zelensky did him the favor.
On Wednesday, the White House declassified an unredacted transcript of the phone conversation.
The transcript revealed that the US president asked Zelensky to work with his personal lawyer and the US attorney general to "look into" the case of Joe Biden, who "went around bragging that he stopped the prosecution" into his son’s possible corruption in Ukraine.
In the course of the phone call, Zelensky assured Trump that Ukraine's next prosecutor general would be "100 percent my person" and would look into the Biden case. The transcript did not show that the US president overtly threatened to withhold financial assistance to Ukraine.
Trump said Wednesday that he did not applied pressure on the Ukrainian president. Zelensky told reporters on the same day at a joint news conference with Trump that he did not feel any pressure during the July phone conversation with his US counterpart.
Trump earlier slammed the impeachment announcement based on his telephone call with Zelensky as a "political witch hunt".