Aleksei A. Navalny’s political allies on Saturday confirmed his death, saying that his mother had received an official notification of it.

Kira Yarmysh, Mr. Navalny’s spokeswoman, said in a statement on X that Russian investigators had transferred Mr. Navalny’s body from a penal colony in the Arctic to the nearby town of Salekhard, where it is being examined.

“We demand for Aleksei Navalny’s body to be released to his family immediately,” Ms. Yarmysh said in her statement.

Ms. Yarmysh is a member of a team of Mr. Navalny’s allies. Working from outside Russia, they continued to carry out his work after his poisoning in 2020 and his subsequent imprisonment, publishing his statements and organizing political events.

Mr. Navalny’s mother arrived on Saturday morning at the penal colony in the Kharp settlement, Ms. Yarmysh said in a video statement. They had to wait for two hours before a prison official came out to say that Mr. Navalny’s body was transferred to Salekhard.

Lawyers and rights groups said the process of retrieving Mr. Navalny’s body might take a few days.

Prison authorities said Mr. Navalny fell unconscious and died after a walk at the penal colony above the Arctic Circle, where he was transferred to at the end of December.

In their statement about his death, Russian prison authorities said that its causes were “being determined.” Local investigators said that they launched a “procedural check” into Mr. Navalny’s death. They said that “a set of investigative and operative measures is being carried out aimed at establishing all circumstances of the incident.”