The number two leader of the Zetas drug cartel -- one of the most violent in Mexico -- has been arrested in northern Nuevo Leon state, officials said. Naval officials said they arrested on Thursday Rigoberto Zamarripa Arispe, known as "Comandante Chaparro" in the community of Cadereyta, after an anonymous tip-off. Last month officials arrested four members of the Zetas cartel suspected of involvement in the August casino bombing in Monterrey that left 52 people dead. Some 45,000 people have been killed in drug-related violence since December 2006, when the government launched a massive military crackdown on powerful drug cartels, which are themselves locked in brutal combat. The Zetas drug gang -- set up by ex-army officers turned hitmen in the 1990s -- are blamed for spreading extortion, kidnappings and murders, particularly around the Gulf of Mexico, as well as corruptly influencing politicians.