Source in Saudi Arabia says American private security contractors are carrying out'interrogations' on princes and billionaires arrested in crackdown 

Detained members of Saudi elite have been hung by their feet and beaten by interrogates, source says

Among those hung upside down are Prince Alwaleed bin Talal, an investor worth at least $7 billion who is being held at Riyadh's Ritz Carlton

Arrests were ordered three weeks ago by Crown Prince Mohammed bin Salman

Source claims mercenaries are from 'Blackwater', a claim also made by Lebanese president

But its successor firm denies it has any operations in Saudi Arabia whatsoever and says its staff abide by U.S. law

Americans who commit torture abroad can be jailed for up to 20 years