U.S. immigration officials are cracking down on illegal immigrant smugglers operating along the U.S.-Mexico border after several recent killings.

Officials said Monday they are assigning an additional 50 special agents to the Phoenix, Arizona, area, where killings have recently been on the rise. Last week, while Mexican President Vicente Fox was visiting Arizona, four Mexicans were killed on a state highway in a shoot-out blamed on rival smuggling gangs.

The Arizona desert has become a prime crossing area for smugglers bringing Mexicans into the United States. Law enforcement officials say the smugglers often kill the immigrants being brought in by rival gangs as part of their turf wars.