Federal agents from the Drug Enforcement Administration inspected three NFL teams on Sunday amid claims by former players that franchises mishandled the use of prescription drugs.
Medical staff of the San Francisco 49ers, the Tampa Bay Buccaneers and the Seattle Seahawks were all questioned by the agents after their respective away matches as part of surprise checks.
The staff were asked for documents of any medical substances in their possession, and were also asked for proof of their licence to practise in their state.
"We have no information to indicate that irregularities were found," NFL spokesman Brian McCarthy told reporters.
DEA spokesman Rusty Payne added: "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."
The investigation comes as part of a follow-up to a lawsuit filed by 1,200 former NFL players into allegations that teams withheld vital information about injuries and also their use of prescription drugs to mask injury pain.
