WASHINGTON- Neither the US National Transportation Safety Board (NTSB) nor the FBI have been invited to join the Egyptian-led investigation into the crash of a Russian airliner in the Sinai Peninsula late last month, officials at the two US agencies said on Thursday.Representatives for the FBI said that the bureau had offered both Egyptian and Russian crash investigators forensic assistance and other investigative help, but as of Thursday its offers had not been accepted.An NTSB spokesman said that for the last several days his agency had been answering technical questions from Egyptian crash investigators on an informal basis. Some questions have related to the plane's engines, manufactured by the US company Pratt & Whitney, the spokesman for the air transport safety agency said.But the spokesman said that no formal agreement had been struck between Egyptian authorities and the United States for the NTSB to participate officially in Egypt's investigation of the crash of Metrojet flight 9268, and that no such agreement is imminent.The NTSB says it is normally asked to participate in investigations of foreign air accidents regardless of where the plane or engines were manufactured. The agency is automatically involved in crashes on US soil. The FBI is often invited in cases where terrorism is a possible cause.The NTSB has only offered its assistance to Egypt, the spokesman added, because that is where the plane crashed and Egyptian authorities have the lead in the investigation. No NTSB personnel are in Egypt to assist authorities there, the spokesman said.