Washington (CNN) A congressional watchdog group is conducting a review to determine whether the Trump administration violated appropriations law when it froze nearly $400 million in aid to Ukraine earlier this year, a decision that is currently at the center of an impeachment inquiry into the President, CNN confirmed Thursday.

Lawmakers on both sides of the aisle were caught off guard when reports of the hold first surfaced in August and the issue remains one of the key areas of focus in the ongoing Democrat-led impeachment probe.

The Government Accountability Office initiated the review after Sen. Chris Van Hollen, a Maryland Democrat, asked the US comptroller last week whether the administration violated the law by holding the funds until "the very last moment" and failing to notify Congress of the decision, according to the office's spokesman Charles Young.

The Wall Street Journal was first to report the review.

"GAO wrote an opinion back in December of last year ... where, as I understand your opinion, it would be illegal for the executive branch to hold money for the very last minute," Van Hollen said during a budget meeting on Capitol Hill, asking the comptroller if that was consistent with his understanding of the law.

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