The lawyers said their new client, whose identity is not known, had grown alarmed by the president’s behavior and was among those interviewed to corroborate the allegations of the original whistle-blower.

Takeaway: The claim of firsthand knowledge suggests testimony that might bolster the case against Mr. Trump, and undermine his main line of defense: that the accusations are based on inaccurate, secondhand information.

What to watch: This week, House members will interview the U.S. ambassador to the E.U., the former ambassador to Ukraine and others as they move to finish their inquiry in the next several weeks.

Next week are several deadlines for top White House officials to produce subpoenaed documents. (Wondering how impeachment works in the U.S.? Read our explainer.)

Related: Mr. Trump’s attorney general, William Barr, traveled to Rome last month with a top federal prosecutor to look into one of his boss’s pet conspiracy theories.