A senior doctor who treated Alexei Navalny before he was airlifted to Germany has “suddenly” died, the hospital announced.

Sergey Maximishin, 55, was the deputy chief physician for anesthesiology and resuscitation at the Omsk emergency hospital, a Thursday statement from the hospital said. A spokeswoman for the hospital told CNN that it appeared Maximishin died from a heart attack, according to “preliminary data.”