The officials said Navalny was targeted for his activism in trying to raise questions about what they called Russian corruption.
Many Western countries say Navalny was poisoned with a nerve agent, but the Kremlin has denied any role in his illness and has said it has seen no proof that he was poisoned.
Ironically, the poison was stored in a can used for baby formula and forgotten in an area of the kindergarten where toddlers usually roam around and play freely.
"Initial reports indicate soldiers consumed this substance, thinking they were drinking an alcoholic beverage," the Army statement added.
It was not immediately clear why he had been detained.
Navalny said his own investigators had checked the findings and that the case was now solved despite the absence of an official investigation in Russia.
The West has demanded an explanation from the Kremlin, which has denied any involvement in the incident and said it has yet to see evidence of a crime.
Various US officials have been targeted by ricin-laced letters in past incidents.
Ricin is found naturally in castor beans but it takes a deliberate act to convert it into a biological weapon.
Germany has asked for the assistance of the global chemical agency in investigating the alleged poisoning of Navalny