Federal drug agents conducted surprise inspections of National Football League team medical staffs on Sunday as part of an ongoing investigation into prescription drug abuse in the league. The inspections, which entailed bag searches and questioning of team doctors by Drug Enforcement Administration agents in cooperation with the Transportation Security Administration, were based on the suspicion that NFL teams dispense drugs illegally to keep players on the field in violation of the Controlled Substances Act, according to a senior law enforcement official with knowledge of the investigation.