In the desert within 35 miles of one of the nation’s largest cities and about 70 miles north of the U.S.-Mexico border, Arizona authorities found a car ablaze with five bodies burned beyond recognition inside.

The location of the smoldering car in a known smuggling corridor and the nature of the crime itself have Pinal County Sheriff Paul Babeu saying that he’s all but certain a violent cartel is responsible.

The area known as the Vekol Valley where the bodies were found has been the scene of shootouts among smuggling groups fighting over loads of drugs or illegal immigrants, high-speed pursuits and major drug seizures. The area is so heavily trafficked that at one point the Bureau of Land Management posted signs warning the public that “may encounter armed criminals and smuggling vehicles traveling at high rates of speed...”

Spokespeople for the Border Patrol and U.S. Immigration and Customs Enforcement confirmed that their agencies were supporting the sheriff’s investigation but declined to answer further questions.

If investigators confirm that a cartel is responsible for the killings, it would be one of the most extreme examples of spillover violence in the U.S., said Eric Olson, associate director of the Woodrow Wilson International Center for Scholars’ Mexico Institute in Washington, D.C.

“This sort of violence is highly unusual,” Olson said. “It’s a horrible, horrible thing, but we don’t see that happening in a lot of places.”

He said there’s no reason to believe that the incident is the beginning of a new pattern of behavior by cartels or that the violence will start affecting more Americans.

“I think it would be more concerning if it were downtown Phoenix,” Olson said. “But the surest way to attract enormous amounts of law enforcement interest is to kill five people and burn their bodies in downtown Phoenix. They’re not that stupid.”