Chastiakov was “a reliable shoulder for me” since Russia launched its full-scale invasion of Ukraine in February 2022, Zaluzhny wrote. “The reasons and circumstances [surrounding his death] will be established during the pre-trial investigation,” he wrote.

Minister of Internal Affairs Ihor Klymenko wrote on Telegram on Monday that the initial investigation showed that Chastiakov “returned home from work with gifts from colleagues, which he started showing to his family.”

“He took out a gift box with grenades inside and began demonstrating one of the munitions to his son,” he said. His son began twisting one of the grenade rings. Chastiakov reportedly took the munition back and pulled the ring himself, Klymenko wrote.

Police found five more of the same type of grenades in the apartment, he wrote, and had identified the colleague who provided the gift. A search in that serviceman’s office uncovered two similar grenades. “Urgent investigative actions are ongoing,” he said.

Earlier, Ukrainian news outlet Ukrainska Pravda reported on Telegram that Chastiakov’s wife said the explosion was caused by a grenade that was inside a gift bag he had brought home.

The outlet reported that the gift was said to include bottles of liquor and glasses shaped like grenades and that the deadly explosion occurred as Chastiakov tried to open it. Photos accompanying the post show debris surrounding gift bags lined up on the floor next to a couch.

In one photo, what appear to be several grenades or grenade-shaped items are scattered across the floor. On social media, some jumped to the conclusion that Chastiakov’s death had been an assassination rather than a mistake.

“I hope the traitor will be found,” one user wrote on Zaluzhny’s Facebook post announcing the death.

“I pray to God that this was a tragic coincidence and not a targeted attack,” wrote Dana Yarova, member of the Defense Ministry’s anti-corruption council.

On Facebook, lawmaker Mariana Bezuhla wrote that “a detailed investigation should be conducted.”

“I would never have thought that Hennadii would die as a result of carelessness on his birthday,” she wrote. “Grenades are issued, not given. Not kept at home.”

Many regard Ukrainian President Volodymyr Zelensky and Zaluzhny as political rivals. In recent days, that alleged discord has garnered more attention after Zaluzhny wrote in an essay for the Economist that both Russia and Ukraine had reached a “stalemate” in the war and that new technology would be necessary to ensure advances. Zelensky publicly disagreed with that assessment.

Zelensky’s office did not respond immediately to a request for comment on the birthday incident.

Kamila Hrabchuk, Serhiy Morgunov, Kostiantyn Khudov and Isabelle Khurshudyan contributed to this report.