PHOENIX  —  Tighter border security is fueling a resurgence of violent attacks on immigrant smugglers in Arizona, according to authorities.

Despite stepped-up efforts to combat illegal immigration, there have been five incidents in the past month involving armed men attacking vehicles possibly transporting undocumented immigrants to drophouses in Phoenix.

The most recent attack occurred Sunday when three armed men hijacked a van and kidnapped five passengers at a stoplight in Ahwatukee Foothills.

The attacks along the Interstate 10 corridor come as apprehensions of undocumented immigrants by the Border Patrol's Tucson Sector, the busiest in the nation, are down 10 percent this fiscal year — the first decrease in several years.

Federal officials and immigrant advocates agree that tighter border security is contributing to the violence as it becomes harder to smuggle people and drugs into Arizona, the main entry point for illegal immigration on the U.S.-Mexican border.

"Smugglers are killing each other for the profits," said Salvador Reza, an immigrant advocate.

The tighter border security has made immigrant loads more valuable and a target for rival gangs, said Alonzo Pena, special agent in charge of Immigration and Customs Enforcement in Arizona.

"That is an unfortunate, or unintended, consequence," Pena added. "When there are tougher interdiction efforts at the border, the (smuggling) price goes up. It's being driven by greed. They do violent things because they are protecting their investment. They will protect turf, and they will resort to violence."

Pena said the attackers could be border gangs known as bajadores, who rip off loads of illegal immigrants from smugglers and hold them for ransom, or they could be rival groups battling to control smuggling routes.

Federal officials are also looking into the possibility that some of the attacks could be violence spilling over from turf battles south of the border between drug traffickers and immigrant smugglers.