Spokespersons for the House Intelligence Committee did not immediately respond to CNBC's requests for comment about the complaint, which spurred Democratic leaders to support launching an impeachment inquiry this week against Trump.

The complaint was delivered by hand to a secure facility in the Capitol building, where multiple lawmakers, including Intelligence Committee leaders Sen. Richard Burr, R-N.C., and Rep. Adam Schiff, D-Calif., have viewed the materials.

A whistleblower's complaint reportedly about a phone call between President Donald Trump and Ukraine President Volodymyr Zelensky was delivered to Congress on Wednesday.

The whistleblower had filed the complaint to the Intelligence Community's internal watchdog, who deemed the allegations "urgent" and "credible."

Multiple media outlets reported that the complaint raised alarms about Trump's call on July 25 with Zelensky, a five-page memorandum of which was made public Wednesday morning. Trump had asked in the call if Zelensky "can look into" allegations of corruption made against former Vice President Joe Biden and his son, Hunter.

But Joseph Maguire, the acting Director of National Intelligence, did not forward the complaint to Congress, which lawmakers have called a violation of the law. On Tuesday, the usually-polarized Senate voted unanimously for Maguire's office to hand over the complaint.

Maguire is scheduled to testify in a public hearing Thursday. The Washington Post reported that Maguire threatened to resign if the Trump administration tried to stop him from testifying freely — but he denied that report after it was published.

Lawyers for the whistleblower have said that their client wants to testify before Congress. Schiff told CNBC that he hopes "to find out soon" about whether the whistleblower can be brought in.

Schiff said after leaving the secure room that the complaint was credible and disturbing. House Intelligence Committee member Mike Conaway, R-Texas, however, said that he did not think the complaint would compromise Trump if it was declassified.

-- Reuters contributed to this report.