Defense Department official Laura Cooper testified in an impeachment hearing Wednesday that members of her staff recently brought her two unclassified State Department emails revealing that the Ukrainian Embassy had asked about U.S. military aid — which had by then been suspended — on July 25.

Why it matters: July 25 is the same day of President Trump's infamous phone call with Ukrainian President Volodymyr Zelensky, though the emails from the State Department came after the morning call. Cooper's testimony suggests the Ukrainians knew there was "some kind of issue" with the security assistance by July, and that they were aware the aid had been suspended by August.

The surprise revelation could potentially undermine a key Republican defense that Zelensky could not have felt pressure from Trump to carry out investigations into his political rivals because the Ukrainian president was not aware of the frozen aid.

Go deeper: Live updates from the hearing