The United States has long been home to violent gangs, from the Mafia to the Bloods and Crips. But recently, US authorities have warned of the dangers of a transnational, ultra-violent gang with its origins in Central America. The FBI has now opened an office in El Salvador to deal with the threat of Mara Salvatrucha, or MS-13. For Assignment, Maurice Walsh travelled to Washington DC's suburbs and San Salvador to take a look at MS-13, dubbed "The Most Dangerous Gang in America."