The investigation into Mr. Giuliani is tied to the case against two of his associates who were arrested this past week on campaign finance-related charges, the people familiar with the inquiry said. The associates, Lev Parnas and Igor Furman, were charged with funneling illegal contributions to a congressman whose help they had sought in removing Ms. Yovanovitch.

Here’s a guide to the Ukraine saga’s growing cast of characters.

A former ambassador to Ukraine speaks

Ms. Yovanovitch told impeachment investigators on Friday that a top State Department official had told her that Mr. Trump had pushed for her removal for months even though her employer, the State Department, believed she had “done nothing wrong.”

She dismissed as “fictitious” accusations that she had been disloyal to Mr. Trump, which she said had been circulated by allies of Mr. Giuliani.

Ms. Yovanovitch, testifying in defiance of State Department orders and at risk of losing her job, delivered a scathing indictment of how the Trump administration conducts foreign policy, warning that private influence and personal gain have usurped diplomats’ judgment and could undermine the nation’s interests.

Ms. Yovanovitch said of her time in Ukraine: “Our efforts were intended, and evidently succeeded, in thwarting corrupt interests in Ukraine, who fought back by selling baseless conspiracy theories to anyone who would listen. Sadly, someone was listening, and our nation is the worse off for that.”