WASHINGTON — The F.B.I. has agreed to help the Russian government with its investigation into the deadly crash of a Russian charter plane in the Sinai Peninsula in Egypt, senior American officials said on Saturday.

Some American officials said that the Russians want help doing a forensic analysis to determine what brought down the Airbus A321-200, while other officials said that the request from the Russians was more general. Although most of the debris is scattered over nearly eight square miles in the desert, some parts of the plane were taken to Russia for analysis.

It is rare for the Russians to make such a request, which was first reported on Friday by CBS News, and some American officials interpreted it as a sign of the challenges facing investigators.

Moreover, although the F.B.I. and its Russian counterpart, the Federal Security Service, or F.S.B., often work together on terrorism issues when they see a common enemy — in Muslim extremist groups like Al Qaeda and the Islamic State, for example — American-Russian relations are at one of their lowest points since the fall of the Berlin Wall.