MOSCOW (AP) — When a powerful blast shook a Russian city near the border of Ukraine residents thought it was an Ukrainian attack. But the Russian military quickly acknowledged that it was a bomb accidentally dropped by one of its own warplanes.

Belgorod, a city of 340,000 about 40 kilometers (25 miles) east of the border, has faced regular drone attacks that Russian authorities blame on the Ukrainian military, but the explosion late Thursday was far more powerful than anything its residents had heard before.

Witnesses reported a low hissing sound followed by a blast that made nearby apartment buildings tremble and threw a car on a store roof. It left a 20-meter (66-foot) -wide crater in the middle of a tree-lined boulevard flanked by apartment buildings, shattering their windows, damaging several cars and injuring two residents. A third person was later hospitalized with hypertension.

Immediately after the explosion, Russian commentators and military bloggers were abuzz with theories about what weapon Ukraine had used for the attack. Many called for a powerful retribution.

But about an hour later, the Russian Defense Ministry acknowledged that the explosion was caused by a weapon accidentally dropped by one of its own Su-34 bombers. It didn’t offer any further details, but military experts said the weapon likely was a powerful 500-kilogram (1,100-pound) bomb.

In Thursday’s blast, the weapon was apparently set to explode with a small delay after impact, to hit underground facilities.

Belgorod Gov. Vyacheslav Gladkov said that local authorities decided to temporarily resettle residents of a nine-story apartment building near the blast while it was inspected to make sure it hadn’t suffered irreparable structural damage.

The explosion in Belgorod followed the crash of a Russian warplane next to a residential building in the port city of Yeysk on the Sea of Azov that killed 15 people. Yeysk hosts a big Russian air base with warplanes flying missions over Ukraine.

Military experts have noted that as the number of Russian military flights have increased sharply during the fighting, so have the crashes and accidents.