The Justice Department has launched a federal investigation into Planned Parenthood’s practices and the sale of fetal tissue.

In a letter first obtained by Fox News, Justice Department Assistant Attorney General for Legislative Affairs Stephen Boyd formally requested unredacted documents from the Senate Judiciary Committee, the same panel that led the congressional probe into the women’s health organization.

“The Department of Justice appreciates the offer of assistance in obtaining these materials, and would like to request the Committee provide unredacted copies of records contained in the report, in order to further the Department’s ability to conduct a thorough and comprehensive assessment of that report based on the full range of information available,” Boyd wrote.

Fox News has learned that last month, the FBI first requested the unredacted documents from the committee.

Fox News is told that Committee Chairman Chuck Grassley, R-Iowa, and Ranking Member Dianne Feinstein, D-Calif., said they needed to receive a letter and be assured that the documents would be used for investigative purposes.

The letter, sent to Grassley and Feinstein on Thursday, is a rare confirmation by the Justice Department of a federal investigation.

“At this point, the records are intended for investigative use only—we understand that a resolution from the Senate may be required if the Department were to use any of the unredacted materials in a formal legal proceeding, such as a grand jury,” Boyd also wrote.

A spokesman for the Senate Judiciary Committee did not respond to Fox News’ request for comment.

In the committee’s final report entitled “Human Fetal Tissue Research: Context and Controversy” published in December 2016, Grassley referred Planned Parenthood and other providers to the FBI for investigation.

Grassley said at the time that the committee has discovered enough evidence that shows how abortion providers had transferred fetal tissue and body parts from aborted fetuses for research by charging amounts higher than they actually cost.

“The report documents the failure of the Department of Justice, across multiple administrations, to enforce the law that bans the buying and selling of human fetal tissue,” Grassley wrote last December urging the Justice Department and FBI to investigate. “It also documents substantial evidence suggesting that the specific entities involved in the recent controversy, and/or individuals employed by those entities, may have violated that law.

Fox News’ Perry Chiaramonte contributed to this report.