The Justice Department is looking into Planned Parenthood's practices related to human fetal tissue, according to a letter obtained by CNN.

The letter -- from the Justice Department's head of its Office of Legislative Affairs -- does not mention Planned Parenthood by name but asks the Senate Judiciary Committee to turn over unredacted documentation supporting a December 2016 report titled "Human Fetal Tissue Research," which purports to describe the organization's practices.

"At this point, the records are intended for investigative use only," Assistant Attorney General Stephen Boyd wrote. "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."

The use of human fetal tissue -- which is used to study a number of diseases -- has proved to be a political flashpoint for decades.

Federal law prohibits the receipt of any "valuable consideration" for fetal tissue, while permitting "reasonable payments" for costs, including "transportation, implantation, processing, preservation, quality control or storage of human fetal tissue."

Senate Judiciary Chairman Chuck Grassley's 2016 report claimed that executive branch agencies had failed to monitor the industry and -- after reviewing 20,000 documents voluntarily submitted by Planned Parenthood and a number of other organizations -- called on the Justice Department to investigate.

"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. "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."

Last month, in response to reports that the FBI had asked the Senate for documents it obtained from abortion providers, Planned Parenthood's vice president of government affairs, Dana Singiser, said: "Planned Parenthood strongly disagrees with the recommendations of the Senate Republican staff to refer this matter to the Justice Department, especially in light of the fact that investigations by three other Congressional committees, and investigations in 13 states including a Grand Jury in Texas, have all shown that Planned Parenthood did nothing wrong."

Singiser added: "Planned Parenthood has never, and would never, profit while facilitating its patients' choice to donate fetal tissue for use in important medical research."

CNN is told that Grassley said in order to turn over the documentation he needed to receive a letter from DOJ explaining that the materials underlying his report would be used for investigative purposes, according a source with knowledge of the discussions.

The Justice Department declined to comment.