The findings from five European governments that Kremlin critic Alexei Navalny was killed with poison are “troubling,” U.S. Secretary of State Marco Rubio said on Sunday.

France, Germany, the Netherlands, Sweden and the United Kingdom said on Saturday that Russia poisoned Navalny with a toxin from a poison dart frog, leading to his death in a penal colony two years ago. Moscow dismissed the accusation.

“We obviously are aware of the report. It’s a troubling report. We’re aware of that case of Mr. Navalny and certainly … we don’t have any reason to question it,” news agency Reuters quoted Rubio as telling reporters at a press conference in Slovakia on Sunday.

Asked why the U.S. did not join the five countries in making a statement, Rubio said: “Those countries came to that conclusion. They coordinated that. … Doesn’t mean we disagree with the outcome.”

Rubio on Sunday began a two-day trip to Slovakia and Hungary, aiming to boost ties with two EU countries that frequently clash with Brussels. He plans to meet with Hungarian Prime Minister Viktor Orbán, who faces an election in April, on Monday.