On Tuesday, the FDA continued its investigation and crackdown on bad actors within the pharmaceutical industry by sending a warning letter sent to drug distributor McKessson. The sternly-worded letter accuses the opioid distrubitor of shipping “illegitimate” product. This is the first time the FDA has taken formal enforcement action to curb abuse.

The letter from the FDA is significant as an indicator of future enforcement action, accusing McKesson of failing to identify, quarantine and investigate drug shipments that had been tampered with. Some of the bad shipments were discovered at three Rite Aid pharmacies in Michigan and included opioids, while other issues were identified with drug shipments for other ailments, such as HIV, cholestoral and heart problems.

Warning letters are usually the first step in any regulatory action the FDA takes against pharmaceutical distributors or manufacturer. The FDA has not ruled out any possible legal action, including an injunction on sales.

Distributors have come under fire for their role in the opioid crisis. McKesson and Cardinal Health are the subject of lawsuits related to the opioid crisis and accountability for a huge influx of prescriptions.

“A distributor’s failure to have systems in place to investigate and quarantine suspect and illegitimate products within their control is a violation of the law. But this is even more concerning given that we’re in the midst of a widespread opioid crisis,” FDA Commissioner Scott Gottlieb said in a statement.

Distributors have come under fire for their role in the opioid crisis. McKesson and Cardinal Health are both subjects of numerous state and local lawsuits alleging they flooded communities with opioids.

“A distributor’s failure to have systems in place to investigate and quarantine suspect and illegitimate products within their control is a violation of the law. But this is even more concerning given that we’re in the midst of a widespread opioid crisis,” FDA Commissioner Scott Gottlieb told CNBC in a statement.

The FDA warning letter gave three examples of the negligence. In each case, McKesson shipped prescription drugs that were supposed to be opioids. In September and October 2016, three Rite Aid pharmacies in Michigan reported receiving bottles oxycodone hydrochloride that were instead filled with naproxen or a combination of naproxen and ciprofloxacin hydrochloride, the FDA said.

Although McKesson did investigate the incidents, the FDA said the company did not go far enough with its investagations. They should have been vigilant in identifying all illegitimate products. McKesson also should have notified its other customers that might have received products from the same batch, the FDA said.

Shortly after the Michigan incidents, in December 2016, Albertsons pharmacy submitted illegitimate product notifications to the FDA for divalproex (a mood stabilizer) and losartan potassium and hydrochlorothiazide (heart medication) it received from McKesson. None of the packaging contained lot numbers or expiration dates.

The FDA also said in June 2016, drugmaker GlaxoSmithKline notified McKesson that a pharmacy reported receiving two sealed bottles of a product labeled as Triumeq (a potent drug used for HIV) that actually contained gemfibrozil (a cholesterol drug). The FDA says McKesson was unable to provide records showing it investigated and identified any other illegitimate products or notified customers, the FDA said.

McKesson has 15 days from the time it receives the letter to outline the steps it has taken to correct the violations flagged by the FDA. In this time frame, they are also supposed to identify and initiate all investigations related to other reports of tampered drugs or incorrect labeling and shipments.