Russia knew in advance about Syria’s chemical weapons attack last week that left at least 86 dead, a senior U.S. official told the Associated Press Monday.

The AP cited an official saying that a Russian-operated drone flew over the hospital as victims of the attack were getting treatment, and that a Russian-made jet subsequently bombed the hospital. American officials believe that this strike against the hospital was done in order to cover up evidence of the chemical weapons attack.

AP reported that the official said the U.S. is still unclear about who was flying the Russian-made jet, but believes that the drone’s presence means that Russia knew about the chemical weapons attack. A Department of Defense spokesman, however, discounted this report Monday evening.

“I’ve seen nothing that corroborates this definitive statement,” Major Jamie Davis told the Daily Caller in a statement. “We continue to review the available intelligence surrounding this incident.”

Updated with DOD statement at 8:56 pm.