The newest threat is the growing presence in the U.S. of the bloodthirsty gang known as the Zetas.
The Zetas are former elite Mexican soldiers trained as anti-drug commandos who are now mercenaries for drug traffickers. They have been linked to hundreds of killings and kidnappings on both sides of the border.
Mexico's leading anti-drug prosecutor, Jose Luis Santiago Vasconcelos, says the Zetas are "extremely violent," and "much feared in the region because of the bloodshed they unleash."
The Zetas willingness to operate within the U.S. represents a departure from standard practice for Mexican cartels. Traditionally, traffickers keep a low profile on U.S. soil in an effort to avoid confrontations with U.S. law enforcement.
So while Mexico looks the other way, the Zetas get bolder. And Americans are in greater danger.