U.S. Secretary of State Marco Rubio described as “troubling” a joint report by five European governments concluding that Russian opposition leader Alexei Navalny was killed with a toxin derived from a poison dart frog, Politico reported.

Axar.az reports that France, Germany, the Netherlands, Sweden, and the United Kingdom said their analyses determined that Russia poisoned Navalny in a penal colony two years ago.

Moscow rejected the accusation.

Speaking at a press conference in Slovakia, Rubio said the U.S. has no reason to question the findings, but noted that Washington did not formally join the joint European statement. He emphasized that this does not mean the U.S. disagrees with the conclusion.

Rubio’s remarks came during a two-day visit to Slovakia and Hungary, where he is meeting leaders including Slovak Prime Minister Robert Fico and Hungarian Prime Minister Viktor Orbán — both leaders known for close ties with President Donald Trump.