KYIV, Ukraine — As deputy foreign minister, it was Olena Zerkal ’s job to read incoming diplomatic cables from embassies around the world. One from Washington caught her eye back in July, she recalled: It said the Trump administration had frozen military aid for Ukraine.

“We had this information,” Ms. Zerkal said in an interview. “ It was definitely mentioned there were some issues.”

The timing of when Ukraine knew of the hold on the military aid is a critical question in the impeachment hearings in Congress. Democrats are trying to build a case that President Trump pressured President Volodymyr Zelensky by withholding the aid and a White House meeting — at the same time he was pressing for a public announcement that Ukraine would investigate his political rivals .

Mr. Trump and his allies have also made the timing issue part of his defense. How could Ukrainian officials feel the pressure of the aid freeze, they ask, if they did not know about it when the White House was pushing them to investigate Vice President Joseph R. Biden Jr., one of Mr. Trump’s biggest challengers in the 2020 American presidential election?