An off-duty Memphis police officer killed his fiancee's ex-husband during a child custody exchange at gas station, the third fatal shooting by an officer this year, authorities said Monday.

The off-duty officer was at the gas station to witness his fiancee picking up her 4-year-old daughter from and her ex-husband Sunday night, Tennessee Bureau of Investigation spokesman Josh DeVine said.

The ex-husband, Luis Philipe Soto and the officer got into an altercation. The situation escalated, and the officer shot Soto, 29, DeVine said. Investigators found what they believe to be Soto's gun near his body, but they were still working to determine if he fired it during the struggle, DeVine said.

It was not immediately clear where exactly the child was during the shooting, but she was present, DeVine said.

"It's our understanding that the child may have been turned away from this incident, so she may not have necessarily witnessed this shooting," DeVine said.

The officer's name and race were not released by the Memphis Police Department or the bureau. Soto was Hispanic.

DeVine said the investigation was in its early stages and many details of the shooting were still unknown, including whether the officer used his service weapon and what sparked the altercation.

Investigators will turn over their findings to Shelby County District Attorney Amy Weirich when the probe is completed. DeVine said the investigation could take weeks.

The bureau is currently investigating two other shootings by Memphis police officers this year. Johnathan Bratcher, 32, was shot by police during a foot chase in a Memphis neighborhood Jan. 27, authorities said.

Alexio Allen, 30, was shot by an officer inside a house in the Raleigh neighborhood on March 23, authorities said.

The bureau began investigating officer-involved shootings in Shelby County with the case of 19-year-old Darrius Stewart. He was killed by Officer Connor Schilling as they fought when the officer tried to arrest the teen on two active warrants during a July 17 traffic stop. Schilling is white, and Stewart was black.

Investigators gave Weirich an 800-page report, and she recommended that a grand jury indict Schilling on charges of voluntary manslaughter and employment of a firearm during the commission of a dangerous felony. The grand jury declined to charge Schilling, who has retired. The department has said Schilling, 27, violated policies for handcuffing techniques and radio procedures on the night of the Stewart shooting.