An Oklahoma reserve police officer who shot and killed a suspect during a weapons-buying sting operation Thursday did so inadvertently after mistaking his own service pistol for a Taser, police said Saturday. The 73-year-old officer, Robert Bates, was put on administrative leave and the Tulsa County Sheriff’s Office is investigating the death.

Bates fired one shot at 44-year-old Eric Harris around 11 a.m. Thursday while police had Harris on the ground, struggling to handcuff him after he fled from them on foot. Harris had tried to sell an undercover officer a 9mm handgun and ammo and was seen “reaching for his waistband area,” just prior to the shooting, officers said.

The operation was being conducted under the auspices of the Tulsa County Violent Crimes Task Force, which deals with drug- and weapons-related crimes. It is not clear if Harris was carrying a weapon at the time besides the handgun he tried to sell to the undercover officer, KTUL Tulsa reported. Harris died in an area hospital shortly after.

“During the rapidly evolving altercation, the reserve deputy had what he believed was his Taser from his tactical carrier and attempted to render aid in subduing the suspect, a sheriff's office news release said. Bates “was attempting to use less lethal force believing he was utilizing a Taser, when he inadvertently discharged his service weapon, firing one round, which struck Harris.”

Bates reportedly immediately realized his mistake and dropped his weapon in shock after firing the round. He is said to be a longtime Tulsa police officer and won reserve deputy of the year in 2011. He received training “in homicide investigation, meth lab identification and decontamination, and other specialized training,” as a part of his service in the violent crimes task force, the sheriff's office said.

Bates was considered an advanced-level reserve deputy, meaning he had at least 320 hours of training and passed yearly lethal and nonlethal weapons exams. Shannon Clark of the sheriff's office said the investigation will determine if Bates’ age was a factor in the case, or if it was attributed to the “rapidly evolving situation."

In 2009, a similar incident happened in Oakland, California, when a transit officer shot and killed 22-year-old Oscar Grant III after the officer supposedly thought he was using a Taser instead of a gun. Johannes Mehserle was found guilty of involuntary manslaughter in a 2010 criminal case over the shooting, but was acquitted on a murder charge. He served one year in prison and was released in 2011. A court in 2014 ruled in favor of Mehserle in a civil suit brought by Grant's family. The family settled a separate suit over the killing earlier.