Unexplained explosions were heard, fires broke out, and a railway bridge collapsed near the Ukrainian border in Russia on Sunday and early Monday morning in three separate incidents, according to Russian officials.

One person was reportedly injured after a large fire engulfed a Russian military facility in the Belgorod region of Russia on Sunday, according to the region’s governor. Belgorod shares a border with northern Ukraine. No explanation was given for the cause of the fire by regional officials or the Russian Defense Ministry. The fire was followed by reports of explosions in the same region on Monday. In another region of Russia bordering Ukraine, a railway bridge collapsed on Sunday in what the governor is calling an act of “sabotage.”