The Internal Revenue Service (IRS) had a written policy instructing agents to alter details of Drug Enforcement Administration (DEA) investigations to hide the trail of tips the agencies exchange, according to a Reuters report released on Wednesday.

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The report says the Justice Department is reviewing an IRS manual published online in 2005 and 2006 that “instructed agents of the U.S. tax agency to omit any reference to tips supplied by the DEA's Special Operations Division, especially from affidavits, court proceedings or investigative files.”

The Special Operations Division of the DEA feeds tips culled from a National Security Agency (NSA) surveillance program to the IRS, which uses the information to investigate potential crimes.

IRS agents were instructed to “recreate” the chain of information to shield the DEA as the source of the information, according to the report.

The DEA contends that the practice is legal, has been used daily for about two decades, and is necessary to protect the agency’s sources and methods.

The NSA program in question is separate from the controversial programs Edward Snowden leaked to The Guardian, according to Reuters.