Mara Salvatrucha, MS-13, or La Mara, as it is called on the streets, started in Los Angeles in the 1980s among Salvadorans who came to the United States to flee a civil war.
It is now a transnational organisation present in Central America and also in the US, where there are large Central American communities. There are, for instance, 1.5 million Salvadorans in the US.
La Mara exists in small pockets within these communities.
There is no hierarchy. While there are some leaders - mostly in El Salvador, sometimes even in jail but still with influence - the gang is organised as loose, semi-autonomous cliques, with loyalty to the clique.
The gang is unique in some respects. In El Salvador and some other Central American countries, it may control territory - over which it battles law enforcers - and deal in drugs and demand protection money.
In the US, while in some cases drugs and protection money are involved, it is not a wealthy gang, analysts say.
In fact, the gang is more like a criminal social phenomenon,the result of a destabilised El Salvador, for which, say analysts, America shares the blame.
Mr Steven Dudley, co-director of InsightCrime, a think-tank that focuses on organised crime in the Americas, told the website Snopes in February: "This is the vicious circle that we've been in since the early 1980s at least, where a US-backed war in El Salvador leads to massive numbers of refugees in Los Angeles and other places who live in very tense, violent, marginalised circumstances, and create means by which they can defend themselves, or feel like their only means of social climbing is through criminal and violent acts."
Arrested and deported, MS-13 members find fertile ground back home in post-conflict, poverty-and violence-torn El Salvador - and others keep coming to the US anyway.
But analysts say there is little evidence to suggest an organised effort to channel MS-13 gang members into the US.
The analysts also maintain that using MS-13 as an example of the downside of illegal immigration unfairly tars all immigrants from Central America.
Arrested and deported, MS-13 members find fertile ground back home in post-conflict, poverty-and violence-torn El Salvador - and others keep coming to the US anyway.
