Senior US military officials say the Pentagon is looking into whether Russia participated in the Syrian chemical weapons attack on a rebel-held town, according to the Associated Press.

A drone belonging either to Russia or Syria was seen hovering over the site of the chemical weapons attack Tuesday after it happened, the officials told The AP.

The unmanned aerial vehicle returned later in the day as people sought treatment at a local hospital, which was bombed a short time later.

The officials say they believe the hospital strike may have been an effort to cover up evidence of the chemical weapons attack.

Asked if the Trump administration was concerned about possible Russian complicity, White House spokesman Sean Spicer on Friday said: “The actions that were taken were clearly against the Assad regime and I’m not going to say anything further than that.”

A Syrian Air Force Su-22 warplane was monitored dropping a chemical weapons bomb that landed in Khan Sheikhun, where 86 people were killed, including 28 children, The Washington Examiner reported.

Two officials who briefed reporters at the Pentagon Friday said the US had no evidence of Russian complicity, but that any leads would be followed up.

“Any implication or lead that would indicate Russian involvement, we’ll investigate that lead,” one official said, the paper reported.

The officials said Russia has failed to control the Syrian government’s use of chemical weapons.

The sources weren’t authorized to speak publicly on the matter and demanded anonymity. They said they’re still reviewing evidence.

In response to the attack, the US launched 59 Tomahawk missiles against Syria’s Shayrat air base on Thursday.