The Federal Air Marshal Service is arguably the least competent law enforcement agency in the country. Naturally they are part of the TSA, which reportedly has a program to make sure air marshals don’t show up tot he airport drunk.
No air marshal has ever stopped a terrorist or hijacker since the service was founded in 1962. Although an air marshal did shoot and kill a US citizen in 2005. If something really bad did happen on a flight and an air marshal was onboard they lack the training to do anything about it.