FILE - In his photo April 4, 2017 file photo, provided by the Syrian anti-government activist group Edlib Media Center, which has been authenticated based on its contents and other AP reporting, shows victims of a suspected chemical attack, in the town of Khan Sheikhoun, northern Idlib province, Syria. A senior U.S. official says the U.S. has concluded that Russia knew in advance of Syria’s chemical weapons attack last week. (Edlib Media Center, via AP)

FILE - In his photo April 4, 2017 file photo, provided by the Syrian anti-government activist group Edlib Media Center, which has been authenticated based on its contents and other AP reporting, shows victims of a suspected chemical attack, in the town of Khan Sheikhoun, northern Idlib province, Syria. A senior U.S. official says the U.S. has concluded that Russia knew in advance of Syria’s chemical weapons attack last week. (Edlib Media Center, via AP)

WASHINGTON (AP) — The United States has concluded that Russia knew ahead of time that Syria would launch a chemical weapons attack last week, a senior U.S. official says.

The official offered circumstantial elements to back up his claim, but no concrete proof. And others in the Trump administration cautioned that final American determination had been made that Russia had advance knowledge of the attack, which killed more than 80 people and prompted retaliatory U.S. cruise missile strikes.

The senior official said Monday that a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment. Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

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The presence of the surveillance drone over the hospital couldn’t have been a coincidence, the senior U.S. official said, arguing that Russia must have known the chemical weapons attack was coming and that victims were seeking treatment

The official, who wasn’t authorized to speak publicly on intelligence matters and demanded anonymity, didn’t give precise timing for when the drone was in the area. The official also didn’t provide details for the military and intelligence information that form the basis of what the Pentagon now believes.

A senior White House official later said there is “no U.S. intelligence community consensus” yet on when Russia might have known of the attack. That official refused to be quoted by name, even though President Donald Trump has criticized media for citing anonymous sources.

The allegation of Russian foreknowledge is grave, even by the standards of the currently dismal U.S.-Russian relations. And it is being made as Secretary of State Rex Tillerson travels to Moscow for talks on Syria and other matters.

Although Russia has steadfastly supported Syrian President Bashar Assad’s government, and they’ve coordinated military attacks together, Washington has never previously accused Moscow of complicity in any attack that involved the gassing of innocent civilians, including children. The former Cold War foes even worked together in 2013 to remove and destroy more than 1,300 tons of Syrian chemical weapons and agents.

Until Monday, U.S. officials had said they weren’t sure whether Russia or Syria operated the drone. The official said the U.S. is now convinced Russia controlled the drone. The official said it still isn’t clear who was flying the jet that bombed the hospital, because the Syrians also fly Russian-made aircraft.

U.S. officials previously have said Russians routinely work with Syrians at the Shayrat air base where the attack is supposed to have originated. U.S. officials say the chemical weapons were stored there and that those elements add to the conclusion that Russia was complicit in the attack.

Last Thursday 59 Tomahawk missiles were fired on the government-controlled base in the United States’ first direct military action against Assad’s forces..