The Pentagon is looking into whether Russia participated or assisted in the April 4 chemical attack in Syria, as well as well as an attack on a local hospital, senior U.S. military officials said Friday.

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

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Officials said a Russian-made drone hovered over the hospital where victims of the chemical attack, which left at least 70 civilians dead and hundreds injured, were being treated. Syrian forces own Russian-made aircraft and drones, making it difficult to determine who controlled the craft.

Five hours later, the drone returned and the hospital was struck by munitions dropped from a separate fixed-wing aircraft.

“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,” the official said.

Officials said the drone and the aircraft that dropped the munitions on the hospital could have been owned by Russian or Syrian forces and that they are investigating the identity.

“Any implication or lead that would indicate Russian involvement, we’ll investigate that lead,” the official said. “We don not know the identity of the aircraft at this time.”

The Pentagon pointed to Syria’s 2013 use of chemical weapons as proof the Russians failed to rein in the Syrian regime.

“Russia stepped in at the time and said they would guarantee the removal and elimination of Syria’s chemical weapons capability. At the time they reported that the Syrians had turned in all of their chemical weapons.”

The Russians “failed to control the activities of their Syrian client,” the official said.

If there’s any evidence or credible accusations on Russian participation, “we’ll follow up on that to the best of our ability.”

“We know the Russian have chemical expertise in country,” the official added. “We can not talk about openly any complicity between the Russians and the Syrian regime in this case, but we’re carefully assessing any information that would implicate the Russians knew or assisted with this Syrian capability.”