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.