WASHINGTON — As the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, Laura K. Cooper was one of multiple Pentagon officials who warned the White House that freezing congressionally approved aid to Ukraine could violate the law.

Ms. Cooper, as well as other officials, repeatedly voiced concern that withholding the aid for a sustained period of time could run afoul of the Impoundment Control Act, which prevents the president from unilaterally deciding against spending money allocated by lawmakers. She also personally fielded two separate phone calls from alarmed Ukrainian diplomats quietly seeking information about the delay.

The security assistance to Ukraine — a total of nearly $400 million — and the timing of its delay is at the heart of the impeachment investigation into whether President Trump abused the power of his office to enlist a foreign government for help in an election. When she testifies Wednesday before the House Intelligence Committee, Ms. Cooper, a career Defense Department official, is expected to provide some insight into efforts to persuade the Trump administration to release the aid.

After a mid-June meeting with Mr. Trump, nearly a month before the aid was frozen, the administration began asking Ms. Cooper detailed questions about the money, she told House investigators last month in a closed-door interview. Those questions appeared prompted in part by an announcement from her office that the Pentagon planned to provide $250 million to Ukraine for training and equipment.