According to Russia, the New Year's rocket attack that killed at least 89 Russian soldiers was caused by soldiers using their cell phones.
Ukraine bombed a college for conscripts in Makiivka, within the seized Donetsk region, just after midnight on 1 January.
According to the Russian military, soldiers' use of illegal cell phones allowed the enemy to pinpoint its target.
Although the number of fatalities is unconfirmed, it is the highest death toll admitted by Russia during the conflict.
Ukraine asserts the number is much higher, claiming 400 soldiers were killed, and 300 were injured in the attack.
At 00:01 local time on New Year's Day, according to Russia, six rocket launches from a vocational college using a Himars rocket system manufactured in the United States; two were shot down.
Lt. Col. Bachurin, the regiment's second-in-command, was among those killed, the defense ministry announced early Wednesday morning via Telegram.
The incident is being investigated by a commission, according to the statement.
He said that it is "already obvious" that the primary reason for the strike was the presence and "mass use" of mobile phones by troops inside the range of Ukrainian weaponry, despite this beings prohibited.
This allowed the enemy to find and calculate military personnel locations to launch a missile attack.
According to the statement, officials found responsible in the probe will be brought to justice, and actions are being taken to prevent similar incidents in the future.
Russia also increased the number of Russian soldiers killed in the strike from 63 to 89, but it is impossible to verify this number. Extremely infrequently does Moscow confirm battlefield casualties?
In September, President Vladimir Putin ordered the partial mobilization of 300,000 male conscripts. At the time, the vocational college was crowded with conscripts. Additionally, ammunition was kept adjacent to the location, which had been reduced to rubble.
Some Russian critics and politicians have blamed the military for incompetence, stating that the troops should have never been provided with such precarious housing.
A former top official of Russia's proxy government in Donetsk, Pavel Gubarev, characterized the decision to keep many soldiers in one building as "criminal negligence."
"If no one is punished for this, then it will only get worse," he said.
Andrei Medvedev, the vice-speaker of the municipal parliament in Moscow, stated that it was foreseeable that the soldiers would be punished rather than the commander who decided to place so many of them in one location.
On Tuesday, President Putin issued an order mandating that the families of National Guard servicemen killed in service get 5 million roubles (£57,000; $69,000).
