Drug Enforcement Administration (DEA) agents surprised National Football League (NFL) team medical staffs on Sunday with inspections. The agents were looking for evidence of drugs dispensed to players illegally to keep them on the field in violation of the Controlled Substances Act, The Washington Post reports.

The DEA agents were looking for signs of drugs that were distributed without prescriptions or labels, or dispensed by trainers instead of doctors. Under federal law, only doctors or nurse practitioners can distribute prescription drugs. It is illegal for a doctor to distribute prescription drugs outside of their geographic area of practice.

The inspections were part of an ongoing investigation into prescription drug abuse in the NFL, the article notes. The agents searched bags and questioned team doctors. One law enforcement official told the newspaper DEA agents inspected multiple teams. The San Francisco 49ers confirmed they were inspected after their game against the New York Giants. The Seattle Seahawks and the Tampa Bay Buccaneers also received an inspection.

DEA spokesman Rusty Payne said the investigation was triggered by a class-action lawsuit filed by more than 1,300 retired players. The players allege the league illegally supplied them with prescription painkillers. They say the drugs numbed their injuries and led to medical complications. Lawyers for the league deny the allegations.

According to the lawsuit, players said they were given unlabeled drugs in dangerous combinations. They allege trainers passed out pills in hotels, locker rooms, or on team planes after games while players were drinking alcohol. The lawsuit states the drugs given to players included painkillers such as Percodan, Percocet and Vicodin, anti-inflammatories such as Toradol, and sleep aids such as Ambien.

In a statement, NFL Physicians Society President Matt Matava said, “The NFL team doctors strive to comply with all regulations in prescribing and dispensing drugs to our patients, the players.”