The incident, which took place in the Republic of Dagestan's capital, Makhachkala, has triggered mass protests of police, who many say run roughshod over the public and act outside the law.
The officer in question was detained for questioning Monday, RIA Novosti said. The officer initially said one of the two men had tried to steal his gun and that's when he fired on them. But witnesses in a cafe said nothing like that happened. The two men appeared to be celebrating something when the shots rang out.
Subsequent allegations hold that the cop and one of the two men had a longstanding feud.
Other police allegations include the abduction, torture and sometimes killing of innocents, including two doctors last year accused of terrorism. One doctor's body was eventually returned to his family beheaded and showing signs he'd been beaten. The other doctor was also beaten while in custody.
Their case began when a woman who'd been dumped by one of the doctors made the terrorism allegations, the doctors say to get even for the failed relationship.