12 Hours. 4 Syrian Hospitals Bombed. One Culprit: Russia.

Physicians for Human Rights, an advocacy group that tracks attacks on medical workers in Syria, has documented at least 583 such attacks since 2011, 266 of them since Russia intervened in September 2015. At least 916 medical workers have been killed since 2011.

The Times assembled a large body of evidence to analyze the hospital bombings on May 5 and 6.

Social media posts from Syria, interviews with witnesses, and records from charities that supported the four hospitals provided the approximate time of each strike. The Times obtained logs kept by flight spotters on the ground who warn civilians about incoming airstrikes and crosschecked the time of each strike to confirm that Russian warplanes were overhead. We then listened to and deciphered thousands of Russian Air Force radio transmissions, which recorded months’ worth of pilot activities in the skies above northwestern Syria. The recordings were provided to The Times by a network of observers who insisted on anonymity for their safety.

The spotter logs from May 5 and 6 put Russian pilots above each hospital at the time they were struck, and the Air Force audio recordings from that day feature Russian pilots confirming each bombing. Videos obtained from witnesses and verified by The Times confirmed three of the strikes.

Recklessly or intentionally bombing hospitals is a war crime, but proving culpability amid a complex civil war is extremely difficult, and until now, Syrian medical workers and human rights groups lacked proof.