An ongoing investigation strongly points to the shooting that killed Agent Nicholas Ivie and injured a colleague as "the result of an accidental shooting incident involving only agents," the FBI said in a statement.
"The FBI is utilizing all necessary investigative, forensic, and analytical resources in the course of this investigation," the statement reads.
Authorities had disclosed little information about the circumstances surrounding the shooting that occurred just before 2 a.m. Tuesday. Investigators have yet to shed light on what might have caused the gunfire eruption, or who might have fired first.
Mr. Ivie was one of three agents who had responded to a tripped ground sensor in a remote region of the state – about five miles north of the US-Mexican border – that is a well-known drug-trafficking corridor.
The wounded agent was treated and released from a hospital, while the third was unharmed.
Border patrol officials on Friday told reporters that Ivie's family was aware that his death was probably accidental.
"I explained to Agent Ivie's family that if the investigation ultimately reaches that conclusion, it changes none of the following facts," said Commander Jeffrey Self of the customs and border protection's joint field command in Arizona.
"That Agent Ivie gave the ultimate sacrifice and died serving his country. That he died in the line of duty and will be honored as such for his final act of service. That Agent Ivie served for more than six years protecting this country against those who threaten our way of life. That he will be remembered by all of us who served alongside him for his character, kindness and loyalty."
Like the 2010 shooting death of Border patrol agent Brian Terry in the southern part of the state, Ivie's killing again trained the spotlight on border security. Several politicians, including Arizona Gov. Jan Brewer, quickly assailed the Obama administration's often-repeated assertion that the border is safer than it has ever been.
On Friday, Homeland Security Secretary Janet Napolitano was in Arizona to meet with Ivie's family and law enforcement officials.
Meanwhile, those who worked with Ivie prepared to honor the fallen agent, who leaves behind a wife and two children.
A public memorial service is scheduled for Oct. 8 in Sierra Vista, Ariz., where he lived with his family.