Four IDF soldiers were injured Tuesday when an explosion occurred inside their tank during a routine training exercise in the southern Golan Heights.

A medical team evacuated the soldiers, members of Division 401′s 9th Battalion, by helicopter to Rambam Hospital in Haifa. Two of the soldiers suffered severe injuries, one was mildly injured, and a fourth was lightly injured as a result of smoke inhalation, a source in the IDF said.

All four were out of mortal danger by midnight, Israel Radio reported.

A senior officer in the armored corps explained that the tank crew was taking part in fire drill when a shell jammed in the tank’s cannon. While the crew was trying to extract the shell, the shell broke, spilling gunpowder in the tank’s interior.

Fifteen minutes after cleaning off the powder, one of the soldiers lay a hot, used gun barrel in the crate containing the broken shell. He then switched to a spare gun barrel and continued firing. The heat from the barrel and the ongoing shooting, combined with the remnants of the gunpowder, apparently set off an explosion, the officer said.

According to the officer, an initial investigation found no signs of negligence in the soldiers’ actions during the incident.

“The soldiers acted in accordance with army procedure,” the officer said. “Instinctively, you place the barrel in the crate; it’s standard procedure. Further details are still unclear and are under investigation.”

The IDF launched a full-scale investigation of the incident.