They train for accuracy with paintball guns inside mock airline cabins. They use bullets designed to stop and expand in flesh to minimize damage to an aircraft.

And they can look like Grateful Dead groupies or supermodels.

The Federal Air Marshal Service sends thousands of armed undercover agents into the skies each day. But now the service is facing more scrutiny than ever after marshals shot a Maitland, Fla., man to death Wednesday at Miami International Airport.

Miami-Dade police are investigating the shooting, the first by marshals since before the terror attacks of 2001.

But it's not the first time the program has come under scrutiny.

Federal reports released since the program's rapid expansion after the Sept. 11 attacks found that background checks of agents were often too lenient, discipline problems existed among marshals and firearms testing was often inconsistent.

Now, the training regimens are in the cross hairs as critics analyze the frantic moments that led up to the marshals' firing on 44-year-old Rigoberto Alpizar on the jetway outside an American Airlines flight bound for Orlando.

However, air-marshal service spokesman Dave Adams said the marshals acted appropriately in line with their training.

Marshals are trained to watch passengers at the gate before they board, keeping an eye on who congregates with whom and who could be capable of hijacking a plane, said one air marshal who insisted on anonymity because they generally are not allowed to speak to the media.