The Department of Defense (DOD) plans to investigate whether or not Russia was involved in Tuesday’s chemical weapons attack in Syria.

Defense officials are not currently aware of any Russian involvement in the attack, which killed 72 and injured potentially hundreds more. However, a Russian-made drone was spotted hovering over the hospital where victims were taken.

“We have no knowledge of Russian involvement in this attack, but we will investigate any information that might lead us in that direction,” said a senior defense official during a background briefing with reporters at the Pentagon Friday. “We’re not done.”

The Russian-made drone returned to the hospital five hours later, the hospital was struck shortly thereafter. It is unclear who was operating the drone, as Syrian government forces are known to operate Russian drones.

“We don’t know why somebody or who struck that. We don’t have positive accountability yet, but the fact that somebody would strike the hospital potentially to hide the evidence of a chemical attack, about five hours after, is a question that we’re very interested in,” said the official.

Approximately 72 people died in the chemical attack, while potentially hundreds more were injured.

It is unclear whether Syrian or Russian forces are responsible for the follow-up attack on the hospital, according to the officials.

Russia helped broker an agreement with its ally Syrian President Bashar al-Assad in September 2013 that was supposed to lead to the destruction of Assad’s chemical weapons stockpile. The agreement came in response to a chemical attack the previous month. According to the terms of the agreement, Assad was required to surrender his stockpile by mid-2014. Russia was tasked with aiding in the stockpile’s destruction.

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