Factbox: Key facts about Mexico's "Zetas" drug cartel

(Reuters) - Mexican marines arrested on Sunday the alleged local head of the feared "Zetas" drug cartel in the city of San Luis Potosi in connection with the killing of a U.S. Immigration and Customs Enforcement agent earlier this month.

Here are some facts about the Zetas:

* The Zetas were founded in the late 1990s by about 40 soldiers who deserted from army special forces to work as muscle for the Gulf Cartel. Their name comes from the radio code for top army commanders.

* The Zetas split from their former bosses in early 2010 and are in the midst of an aggressive expansion drive, recruiting from Guatemala and Texas and co-opting existing gangs to do their dirty work.

* The group is headed by Heriberto "The Executioner" Lazcano, who is said to have a huge arsenal of grenades, automatic weapons and even rocket launchers.

* The Zetas are known for their brutality and are blamed for some of the worst atrocities in Mexico's drug war, including the murder of 72 migrants in August and the killing of a leading gubernatorial candidate in June.

* Mexican officials say the Zetas could have as many as 10,000 members across Mexico, Central America and the United States, more than other Mexican cartels -- although the bulk are likely small-time criminal affiliates who pay tribute to the gang rather than full-blown members.

(Compiled by Krista Hughes; Editing by Paul Simao)