Three trainers and a clocker are facing federal fraud and bribery charges over raceday medications and attempts to fix races at Penn National Racecourse in Pennsylvania.

The three trainers - Patricia Rogers, Sam Webb, and David Wells - have been charged with administering illegal raceday medications.

The clocker, Dan Robertson, was said to have taken payments from trainers and owners to falsify trackwork times. The indictment also said some of the horses for which he recorded times had not worked at the track.

If convicted, the trainers could receive maximum sentences of 25 years in prison.

The Daily Racing Form reported the arrests came after a four-year FBI investigation.

The charges were brought against Rogers and Webb after both were caught attempting to administer unidentified substances to horses on a raceday earlier this year.

Wells is most famous for training Rapid Redux, a popular gelding who was retired in 2012 after setting a modern day United States record of 22 straight wins.

The US attorney said in a release the charges were based on the fact that Penn National accepts interstate wagers on its races, making the illegal administration of any medication a federal crime.

A statement from Penn National said it had suspended track employee Robertson on Friday.

The indictment says the charge against Webb is based on an incident on May 2 of this year when he was found by track security in a stall with a horse, Papaleo, while in possession of hypodermic syringes, needles, and bottles of medications.

The indictment against Rogers states she was found in the stall of a horse entered to race on August 2 when she was observed injecting or attempting to inject a substance into the horse.

Raceday medication is prohibited in most states with the exception of furosemide, the anti-bleeding medication more commonly known as Lasix.