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."