From the linked article:
"Kyrylo Budanov, chief of the Main Directorate of Intelligence (HUR) of Ukraine’s Ministry of Defense has said that Russian opposition leader Alexei Navalny died of a blood clot.
“I may disappoint you, but as far as we know, he indeed died as a result of a blood clot. And this has been more or less confirmed,” Budanov told journalists on the sidelines at the “Ukraine. Year of 2024” forum on Sunday.
https://www.kyivpost.com/post/28630