Russian police detained Kremlin critic Alexei Navalny at passport control after he flew home to Russia on Sunday, his lawyer, the prison service and Reuters witnesses said.

It was the first time Navalny has been back home since he was poisoned last summer. His plane was diverted to another Moscow airport at the last minute in an apparent effort by authorities to thwart journalists and supporters greeting him.

Navalny was detained when he showed his passport to border guards before formally entering Russia, Reuters witnesses said. His wife, Yulia, his spokeswoman and his lawyer were allowed to enter Russia.

It was not immediately clear why he had been detained.

But the Russian capital’s prison service had said beforehand it would do everything to arrest him once he returned, accusing him of flouting the terms of a suspended prison sentence for embezzlement, a 2014 case he says was trumped up.