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Read national security official Alexander S. Vindman's prepared remarks to Congress

In his prepared remarks to Congress, Alexander S. Vindman says that Trump undermined U.S. national security during a July call with Ukraine's president.

The top Ukraine expert on the National Security Council is expected to testify to Congress on Tuesday that he was in the White House situation room listening to President Donald Trump's July 25 phone call with Ukraine's president.

Lt. Col. Alexander S. Vindman says in his prepared opening remarks that he was so alarmed by what he heard Trump say on the call that he alerted his superiors and legal counsel at the NSC.

Vindman says in the written statement that it was the second time he raised concern over the administration's apparent attempt to secure a commitment from Ukrainian President Volodymr Zelenskiy to investigate Trump's political rivals in exchange for military aid.

Read Vindman's full written opening statement as prepared.