Ukrainian prosecutors announced on Tuesday that they had opened an investigation into a bizarre explosion at a birthday celebration that killed an aide to Ukraine’s top military commander, in what the authorities portrayed as a tragic accident.

The special defense prosecutor’s office of Ukraine’s central region said that the aide, Maj. Gennadiy Chastyakov, had returned to his home on the outskirts of Kyiv on Monday with birthday gifts from colleagues that included a box containing six grenades. The prosecutor’s office said that one of the grenades had been picked up by the major’s son.

While taking the grenade from the boy, “the officer pulled the ring, which caused the explosion,” the office said in a statement.

Prosecutors said that Major Chastyakov, an aide to Gen. Valery Zaluzhny, commander in chief of the Ukrainian military, had been killed on the spot by the explosion. The blast also seriously wounded the son, 13, and slightly injured the major’s 11-year-old daughter, they added. There was no explanation of why someone would give the major grenades as a present or why he would have pulled the ring.