Over a 26-year career, the trainer Steve Asmussen has built one of horse racing’s largest and most successful operations. He ranks second in career victories, with more than 6,700; has earned more than $214 million in purses, the fifth most in thoroughbred racing; and was recently included on the National Museum of Racing’s Hall of Fame ballot.

But People for the Ethical Treatment of Animals, after conducting an undercover investigation, has accused Asmussen and his top assistant trainer, Scott Blasi, of subjecting their horses to cruel and injurious treatments, administering drugs to them for nontherapeutic purposes, and having one of their jockeys use an electrical device to shock horses into running faster. As a result of its findings, PETA filed complaints with federal and state agencies in Kentucky and New York on Tuesday, saying Asmussen “forced injured and/or suffering horses to race and train.”

The undercover inquiry was conducted by a PETA investigator who worked for Asmussen for four months in the spring and summer of 2013 at Churchill Downs in Louisville, Ky., and Saratoga Race Course in Saratoga Springs, N.Y.

The investigator used a hidden camera to record more than seven hours of video that showed mistreatment of the horses to be widespread and cavalier. In addition, PETA produced a 285-page report about Asmussen’s operations that consisted of the investigator’s notes, medical documents and reports from veterinarians who reviewed the videotape.