The San Francisco 49ers were one of at least three NFL teams subjected to inspection by federal agents Sunday. Reuters/Brad Penner-USA TODAY Sports

The Drug Enforcement Administration subjected the medical staffs of at least three visiting NFL teams to unprecedented investigations Sunday to determine if their policies were in compliance with the federal Controlled Substances Act. No arrests were made, but the ongoing investigation could eventually result in criminal charges if any of the league’s training staffs are found to have mishandled prescription drugs within the last five years, the Associated Press reported.

“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 Drug Enforcement Administration has a responsibility under the Controlled Substances Act to ensure that registrants who possess, prescribe and dispense controlled substances are following the law,” DEA spokesman Rusty Payne said Sunday in a statement.

The federal inspections were conducted without search warrants and were not criminal in nature, CBS Sports reported. Payne declined to speculate on how the DEA would proceed if violations were found, but a source said the organization would make note of any discrepancies.

Federal agents checked team medical bags for controlled substances and interviewed doctors regarding their protocols and procedures relating to the administration of painkillers to injured players, the Washington Post reported. Representatives from the San Francisco 49ers, Tampa Bay Buccaneers and Seattle Seahawks each confirmed team officials met with the DEA.

“Our teams cooperated with the DEA today and we have no information to indicate that irregularities were found,” NFL spokesman Brian McCarthy said in a statement.

The checks targeted visiting teams to determine if NFL medical staffs maintained proper procedure while way from the locations where they are legally registered to dispense pain medication. Officials suspect NFL teams regularly violate federal laws, a federal law enforcement source told ESPN’s “Outside The Lines.”

“NFL doctors are not obtaining a separate registration where they are administering controlled substances to NFL players. They are administering in different states and treating players at hotels and stadiums outside of their registered location with the DEA,” the source said.

A pending lawsuit filed on behalf of more than 1,200 former NFL players triggered the DEA’s NFL raids, reports said. The plaintiffs, including former Chicago Bears quarterback Jim McMahon, allege the NFL and its trainers illegally prescribed painkillers such as Vicodin and Percocet to mask pain while deliberately misleading players about the nature of their injuries, resulting in serious long-term health effects and, in some cases, drug addiction.

The lawsuit represents a range of players who played in the league from 1968 to 2012. Federal drug violations that occurred prior to 2009 fall outside the five-year statute of limitations and cannot be prosecuted, but any violations within the last five years could merit criminal charges.

This isn't the first time the DEA has investigated an NFL medical official for potential prescription drug violations. In 2010, David Chao, then team doctor for the San Diego Chargers, was reviewed as a result of allegations he wrote 108 drug prescriptions to himself. Chao was eventually cleared, but stepped down from his position in 2013 amid pressure from the NFL Players' Association, U-T San Diego reported.

The federal painkiller investigation is the latest in a series of events that generated public criticism toward the NFL in recent months. Concussion-related health issues experienced by former players led the NFL to agree this summer to a $765 million settlement with the more than 4,000 individuals who participated in a class-action lawsuit.

Later, NFL Commissioner Roger Goodell and other league officials were publicly lambasted for administering light punishments to NFL players accused of domestic violence. Former Baltimore Ravens running back Ray Rice, Minnesota Vikings running back Adrian Peterson and Carolina Panthers defensive end Greg Hardy were each arrested for violence against a significant other or child, with only Rice receiving a suspension for his actions. The outrage led Goodell to announce major changes to the NFL's disciplinary policy regarding domestic violence, with more changes expected by February.