This is where "Parallel construction" comes in:

To keep the program secret, the DEA sought not to use the information as evidence in criminal prosecutions or in its justification for warrants or other searches. Instead, its Special Operations Division passed the data to field agents as tips to help them find new targets or focus existing investigations, a process approved by Justice Department lawyers. Many of those tips were classified because the DEA phone searches drew on other intelligence data.

That practice sparked a furor when the Reuters news agency reported in 2013 that the DEA trained agents to conceal the sources of those tips from judges and defense lawyers.

The NSA apparently learned its tricks from the DEA.

The similarities between the NSA program and the DEA operation established a decade earlier are striking – too much so to have been a coincidence, people familiar with the programs said. Former NSA general counsel Stewart Baker said, "It's very hard to see (the DEA operation) as anything other than the precursor" to the NSA's terrorist surveillance.

The OIG began an investigation of the DEA's use of administrative subpoenas and "parallel constructin" a while ago. According to this page, it's still ongoing.

The OIG is examining the DEA’s use of administrative subpoenas to obtain broad collections of data or information. The review will address the legal authority for the acquisition or use of these data collections; the existence and effectiveness of any policies and procedural safeguards established with respect to the collection, use, and retention of the data; the creation, dissemination, and usefulness of any products generated from the data; and the use of “parallel construction” or other techniques to protect the confidentiality of these programs.