WASHINGTON—At least one additional whistleblower with firsthand knowledge of the circumstances around President Trump’s July call with his Ukrainian counterpart has come forward, according to lawyers representing both the individual and the CIA officer whose initial complaint helped spark an impeachment inquiry.

The existence of a second whistleblower comes as Mr. Trump repeatedly has sought to attack the credibility and motive of the first individual, whose whistleblower complaint in August details efforts by the president to pressure Ukraine to investigate a political rival.

“I can confirm that my firm and my team represent multiple whistleblowers,” Andrew Bakaj, the lead attorney for the first whistleblower, wrote in a tweet. “No further comment at this time.”

Mark Zaid, another attorney representing the initial whistleblower, said that a second whistleblower, also an intelligence official, has come forward with information about some of the allegations described in the initial complaint.

That person has been interviewed by the intelligence community inspector general, Michael Atkinson, triggering whistleblower protections as provided by federal law, but hasn’t filed a separate formal whistleblower complaint, Mr. Zaid said. ABC reported on Sunday that the legal team was representing a second whistleblower. Mr. Atkinson’s office didn’t respond to a request for comment.