A preliminary investigation has found friendly fire likely was to blame in a shooting that killed one federal agent and wounded another along the Arizona-Mexico border, the FBI said Friday, shaking up the probe into an incident that reignited the political debate over border security.

“There are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J. Ivie and the injury to a second agent was the result of an accidental shooting incident involving only the agents,” FBI Special Agent in Charge James L. Turgal Jr. said in a statement.

Turgal didn’t elaborate on the agency’s conclusions but said the FBI is using “all necessary investigative, forensic and analytical resources” as it investigates the Tuesday shooting about five miles north of the border near Bisbee.