Washington (CNN) The top Democrat on the Senate Foreign Relations Committee on Tuesday sent letters to three Trump administration officials requesting information about President Donald Trump's July call with Ukraine's President amid escalating scrutiny over the conversation.

The letters from Sen. Bob Menendez -- sent to acting White House chief of staff Mick Mulvaney, Secretary of State Mike Pompeo and Inspector General Steve Linick -- come after the New Jersey Democrat made clear on the House floor Monday that he would seek more information on the call as House Democrats near a "tipping point" on impeachment in the wake of the Trump administration blocking a related whistleblower complaint's release to Congress.

On Friday, CNN reported Trump pressed Ukrainian President Volodymyr Zelensky in a July 25 call to investigate former Vice President Joe Biden's son, Hunter, according to a person familiar with the situation. That call was part o f the whistleblower complaint submitted to the Intelligence Community Inspector General, another person familiar with the situation told CNN. Trump on Sunday acknowledged that he discussed Biden with Zelensky but has denied doing anything improper.

There is no evidence of wrongdoing by either Joe or Hunter Biden.

In his letter to Linick, Menendez requests that the Inspector General "immediately open an investigation" into the Trump administration's withholding of foreign military assistance to Ukraine and whether it was tied to the whistleblower complaint.

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