This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

(CNN) — A leading Senate Democrat is calling for an independent investigation into whether the Trump administration improperly classified key documents that were sent to Congress for political reasons, including the transcript of President Donald Trump’s July 25 call with his Ukrainian counterpart and the War Powers justification for the killing of Iranian Commander Qasem Soleimani.

In a letter sent Thursday to Gene Dodaro, the Comptroller General of the United States, Sen. Chris Murphy, specifically requested that the Government Accountability Office conduct a comparative review of documents to ensure classification levels are consistent with the nature of material held in the executive branch, according to a copy of the correspondence obtained by CNN.

The GAO is an independent federal watchdog group. It determined last month that the Trump administration had broken the law when it withheld the security aid to Ukraine last year, which had been appropriated by Congress.

“My firm belief is that the Trump administration has regularly abused classification rules to hide information from the public. This is an unacceptable and unconstitutional political tactic,” Murphy said in a statement to CNN.

“I’ve asked the GAO to conduct a review of the classified material in the Senate to make sure it’s not overly or unnecessarily classified. We’ve seen the administration classify information just for political purposes recently and the American people deserve better. I look forward to GAO’s findings,” he added.

“The United States Office of Senate Security is in possession of a number of classified documents provided by the executive branch departments and agencies. Much of this information appears to be marked at classification levels that are properly aligned with the nature of the information contained in them. However, some documents contain information that is classified at a level that appears inconsistent with the nature of the material,” the Connecticut Democrat wrote.

“I request that GAO compare a sample of classified documents held by the Office of Senate Security to the original classified versions an underlying source materials held by the respective executive branch departments and agencies to determine the extent to which the classification levels are the same,” he added.

Murphy’s letter comes one day after Trump’s Senate impeachment acquittal — a ruling that was handed down despite acknowledgment from many Republicans that the President’s actions toward Ukraine were improper.

The White House released a partial transcript of Trump’s now infamous July 25 call with Zelensky in September as the whistleblower complaint was made public.

The whistleblower alleged that White House officials tried to “lock down” all records of that July conversation with the Ukrainian leader and the full transcript has not been released. Trump has repeatedly insisted the call was perfect despite bipartisan acknowledgment that it was not.

Documents related to Pence’s September call with Zelensky have also not been released though it was addressed in witness testimony during the House proceedings.

While the Senate voted Wednesday to acquit the President, the complete story of what happened between Trump and Ukraine still hasn’t been told.

Murphy’s request also relates to classified documents outside of the impeachment inquiry, including the War Powers notification that was sent to Congress in January following the strike that killed Soleimani and another unsuccessful operation targeting an Iranian military official in Yemen.

That notification cites Article II and the 2002 Authorization for Use of Military Force (AUMF) as legal justifications but does not make an explicit connection between each individual action and a specific authority, sources told CNN last month.

Democrats have questioned the administration’s motives for keeping the notification classified, particularly as they say officials have struggled to articulate why ordering the military action without informing Congress ahead of time was lawful.