The standard view of the origin of Central American gangs goes as follows:
"The Maras have their roots in Southern California, where young men seeking refuge from Central America's civil wars formed violent gangs on the streets of Los Angeles and its suburbs in the 1980s. Gang members later deported from the U.S. re-established their violent organizations in their native countries of El Salvador, Guatemala and Honduras" (http://tinyurl.com/p29vac7). This is not a wrong view, but it is incomplete and potentially distorting.
Those of us who grew up in the streets of Guatemala, or Central America for that matter, know full well what a "mara" was in the 1970s, that is, before mass migrations and deportations to and from the US began in the 1980s and 1990s. In the 1970s, in fact, the word "mara" entered the vocabulary of young people and permanently redefined the cultural zeitgeist of that generation. A mara was, first of all, a group or network of friends, in many cases from the same general neighborhood although it could also include friends from other "zones", essentially a group of young people hanging out together and using light drugs. The more these groups grew closer together in particular neighborhoods, the stronger the ties and the more territorial they became. It wasn't unusual the find one "mara" coming to another mara's hood and engaging in fights over turf, "girls", and just standing or status. These fights were not brutal and did not yet involve firearms or other deadly weapons, but they were already starting to go in that direction. In fact, the word "mara" – like the word "marea" – often implied the wave, the way to go, going with "la mara". Till it became a matter of being and belonging: my "mara", "soy de la mara", "es la mara".
There were other words becoming available at the time as potential alternatives or replacements for the word "mara". Brosa was one of them, but it is not a word you'll find in your standard Spanish dictionary or even the Real Academia's. The word brosa had several roots including the Spanish word for hacha (http://tinyurl.com/nve55b7) and a word for "montón" (lots, many). The word broza (with a z) also means an indistint grouping of things. This was, at any rate, a word commonly used to describe a group of young people moving a like a wave: "alli viene la brosa" or "el montón de gente" or, eventually, "la mara".
This is how "las maras" were actually born in the 1970s. And it is into this culture that young Central American deportees will arrive in the 1980s and 90s giving the name "mara" its current violent connotations.