Who is attacking Russia’s bases in Syria? A new mystery emerges in the war.

A series of mysterious attacks against the main Russian military base in Syria, including one conducted by a swarm of armed miniature drones, has exposed Russia’s continued vulnerability in the country despite recent claims of victory by President Vladimir Putin.

The attacks have also spurred a flurry of questions over who may be responsible for what amounts to the biggest military challenge yet to Russia’s role in Syria, just when Moscow is seeking to wind its presence down.

In the most recent and unusual of the attacks, more than a dozen armed drones descended from an unknown location onto Russia’s vast Hmeimim air base in northwestern Latakia province, the headquarters of Russia’s military operations in Syria, and on the nearby Russian naval base at Tartus.

Russia said that it shot down seven of the 13 drones and used electronic countermeasures to safely bring down the other six. It said no serious damage was caused.

The drone attack, however, came less than a week after two Russian servicemen were killed in a sustained mortar assault on the same base, which appears to have caused some damage to Russian military assets.

Read the whole story from The Washington Post.