Nine suspects have already been charged in the investigation, due to end in September, the prosecutor said.
Dmitry Muratov, the editor-in-chief of the Novaya Gazeta newspaper which Politkovskaya worked for, dismissed the news as a prosecutorial manoeuvre.
Muratov said the investigation had already announced last year it knew the name of the killer.
He said that the prosecutors now wanted to keep one of the accused, Colonel Pavel Ryaguzov of the Federal Security Service, in prison by saying someone else was still at-large.
Ryaguzov is in a detention centre on charges of abusing his office.
Investigators have alleged that Ryaguzov gave Politkovskaya's address to another suspect, who in turn passed the information on to the killer.