The whistleblower's complaint alleged that the president abused the power of his office to "solicit interference from a foreign country" in next year's U.S. election.
Schiff said Sunday that does not make the complaint less credible because key allegations in it were confirmed by a rough transcript of the Trump-Zelensky call as well as other sources.
Schiff did not say whether he plans to subpoena that information.
The Ukraine scandal cast a shadow over Trump's presidency just months after the conclusion of Special Counsel Robert Mueller's investigation of whether the Trump campaign worked with Russian Federation to help him win the 2016 presidential election.
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American Taylor Fritz , in for the injured Nick Kyrgios , then extended his team's lead by beating Dominic Thiem 7-5 6-7 (3) 10-5. The 38-year-old Federer had won his last two matches against Kyrgios in straight sets, but dropped the opener in a tie-break.
The president's adviser charged the individual, who The New York Times reported was on detail to the White House from the Central Intelligence Agency, is part of a group of unelected bureaucrats trying to take down Mr. Trump.
"When President Trump asks Ukraine to investigate corruption the Democrats want to impeach him and their media lapdogs fall in line", the narrator of the ad says.
Pelosi said she remains focused on the facts, even if they eventually absolve Trump.
In addition, 55 percent of American adults believed the inquiry into Trump was necessary, while 45 percent said it was unnecessary.
The impeachment inquiry has cast a new pall over Mr Trump's presidency just months after he emerged from the shadow cast by Mr Mueller's investigation.
But Volker could be a significant figure in the inquiry because of his potential knowledge of Giuliani's efforts, which Giuliani has claimed without corroboration had come at the behest of the State Department.
The whistleblower said multiple White House officials reported being "deeply disturbed" by the president's call with Zelenskiy, "because of the likelihood, in the officials' retelling, that they had witnessed the President abuse his office for personal gain".
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Giuliani has said the State Department helped arrange meetings with aides to Zelenskiy, posting a copy of a text message from a US diplomat on Twitter.
The whistleblower's complaint was deemed credible by the inspector general of the US intelligence community and the acting director of national intelligence told lawmakers the person "acted in good faith" and 'did the right thing'.
Schiff said his panel is moving forward with its impeachment inquiry into Mr. Trump and will hear from the inspector general again this week, as well as five state department officials.
Appearing on ABC News This Week, the Democrat from California said House lawmakers had settled an agreement with the whistleblower's lawyers to ensure that there are precautions to protect his or her identity during the testimony.
Late on Thursday, Mr Trump denounced people who might have talked to the whistleblower as "close to a spy" and suggested they engaged in treason, an act punishable by death.
Three House committees subpoenaed Secretary of State Mike Pompeo on Friday to hand over documents about his agency's involvement.
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India had proposed a draft document in 1996 on the "Comprehensive Convention on International Terrorism" at the General Assembly. Others backed Trump, saying it is unnecessary to hype on his comments as he was trying to honour Prime Minister Modi.