Federal drug enforcement agents showed up unannounced Sunday to check at least three visiting NFL teams' medical staffs as part of an investigation into former players' claims that teams mishandled prescription drugs.

There were no arrests, Drug Enforcement Administration spokesman Rusty Payne said Sunday. The San Francisco 49ers' staff was checked at MetLife Stadium in East Rutherford, New Jersey, after they played the New York Giants. The Tampa Bay Buccaneers' staff was checked at Baltimore-Washington International airport after playing the Redskins. The Seattle Seahawks, who played at Kansas City, confirmed via the team's Twitter account that they were spot-checked as well.

The operation was still ongoing, and other teams may be checked later Sunday, Payne said.

"DEA agents are currently interviewing NFL team doctors in several locations as part of an ongoing investigation into potential violations of the (Controlled Substances Act)," Payne said.

The spot checks were done by investigators from the federal DEA. They did not target specific teams, but were done to measure whether visiting NFL clubs were generally in compliance with federal law. Agents requested documentation from visiting teams' medical staffs for any controlled substances in their possession, and for proof that doctors could practice medicine in the home team's state.

"Our teams cooperated with the DEA today and we have no information to indicate that irregularities were found," NFL spokesman Brian McCarthy said in an email.

The nationwide probe is being directed by the U.S. Attorney's Office for the Southern District of New York -- where the NFL is headquartered -- but involves several U.S. attorney's offices.

The investigation was sparked by a lawsuit filed in May on behalf of former NFL players going back to 1968. The number of plaintiffs has grown to more than 1,200, including dozens who played as recently as 2012. Any violations of federal drug laws from 2009 forward could also become the subject of a criminal investigation because they would not be subject to the five-year statute of limitations.

"This is an unprecedented raid on a professional sports league," said Steve Silverman, one of the attorneys for the former players. "I trust the evidence reviewed and validated leading up to this action was substantial and compelling."

Federal prosecutors have conducted interviews in at least three cities over the past three weeks, spending two days in Los Angeles in late October meeting with a half-dozen former players -- including at least two who were named plaintiffs in the painkillers lawsuit, according to multiple people with direct knowledge of the meetings who spoke on the condition of anonymity because prosecutors told them not to comment on the meetings.

The lawsuit alleges the NFL and its teams, physicians and trainers acted without regard for players' health, withholding information about injuries while at the same time handing out prescription painkillers such as Vicodin and Percocet, and anti-inflammatories such as Toradol, to mask pain and minimize lost playing time. The players contend some teams filled out prescriptions in players' names without their knowledge or consent, then dispensed those drugs -- according to one plaintiff's lawyer -- "like candy at Halloween," along with combining them in "cocktails."

Several former players interviewed by The Associated Press described the line of teammates waiting to get injections on game day often spilling out from the training room. Others recounted flights home from games where trainers walked down the aisle and players held up a number of fingers to indicate how many pills they wanted.

The controlled substance act says only doctors and nurse practitioners can dispense prescription drugs, and only in states where they are licensed. The act also lays out stringent requirements for acquiring, labeling, storing and transporting drugs. Trainers who are not licensed would be in violation of the law simply by carrying a controlled substance.

The former players have reported a range of debilitating effects, from chronic muscle and bone ailments to permanent nerve and organ damage to addiction. They contend those health problems came from drug use, but many of the conditions haven't been definitively linked to painkillers.

The lawsuit is currently being heard in the northern district of California, where presiding judge William Alsup said he wants to hear the NFL Players Association's position on the case before deciding on the league's motion to dismiss. The NFL maintained that it's not responsible for the medical decisions of its 32 teams. League attorneys also argued the issue should be addressed by the union, which negotiated a collective bargaining agreement that covers player health.

The DEA investigation comes during a turbulent time for the NFL.

The league is still weathering criticism over its treatment of several players accused of domestic violence and just wrapped up an arbitration hearing involving Ravens running back Ray Rice, who is contesting the length of his suspension. The league has hired former FBI director Robert Mueller III to investigate its handling of the Rice case.

The NFL is also trying to finalize a $765 million class-action settlement reached in August 2013 over complaints by thousands of former players that the NFL concealed the risk of concussions.