The Government Accountability Office (GAO) is looking into the Trump administration's hold on nearly $400 million in military aid to Ukraine to see if the freeze, which is at the center of House Democrats' impeachment inquiry, violated appropriations law.

Lawmakers are investigating whether or not Trump used the hold on the congressionally approved aid as leverage in an attempt to get Ukrainian President Volodymyr Zelensky to publicly announce that he would investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE's son, Hunter Biden.

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The review of the administration's actions on Ukraine from the GAO, Congress's top nonpartisan watchdog, was initiated by Sen. Chris Van Hollen Christopher (Chris) Van HollenCongress must finish work on popular conservation bill before time runs out Democrats fear Russia interference could spoil bid to retake Senate Mid-Atlantic states sue EPA over Chesapeake Bay pollution MORE (D-Md.) last week at a Senate Budget Committee hearing.

In the hearing, Hollen asked Gene Dadaro, the U.S. comptroller general, if appropriations law had been broken by the Trump administration not formally alerting Congress of the freeze.

"At Senator Van Hollen’s urging, the GAO is looking into this critical issue — an important development following this troubling move by the administration," Bridgett Frey, spokeswoman for Van Hollen, told The Hill on Thursday.

Trump has repeatedly stated that the release of the aid to Ukraine was not conditional, citing concerns about corruption inside the Ukrainian government as the reason.

However, the Defense Department had reportedly already confirmed that Ukraine was meeting expectations when it came to implementing counter-corruption measures.

Concerns were raised by the Pentagon, which conducted its own legal review of the freeze, that if the aid wasn't released by the end of September, the end of the fiscal year, it could expire.

Rachel Semmel, a spokeswoman for the Office of Management and Budget, told The Wall Street Journal that the administration followed the correct procedures in holding the aid.

"As has been well documented, we fully complied with the law and decades of precedent with respect to these funds," Semmel said.

The aid was finally released in mid-September amid bipartisan pressure.

The GAO's probe was first first reported by the Journal.

The Hill has recached out to the White House for comment.

—Updated at 3:58 p.m.