SHARM EL-SHEIKH, Egypt -- The leading theory since last week has been that a bomb caused the Airbus A321 to break up over Egypt's restive Sinai Peninsula, and now U.S. Intelligence officials say there is a greater focus on how it happened and who is responsible, reports CBS News' justice and homeland security correspondent Jeff Pegues.
As CBS News reported on Sunday, U.S. Intelligence officials believe it is "likely" a bomb brought down the Russian Metrojet plane. killing the 224 people on board.
Based on information from multiple sources, Pegues reports that investigators seem to be zeroing in on the Islamic State of Iraq and Syria (ISIS or ISIL) Sinai branch. A counterterrorism official CBS News contacted would not comment on the record about the cause of the crash but said, "ISIL's Sinai branch is one of the group's most active and potent ISIL affiliates."
The group has been locked in a protracted "insurgency against Egyptian authorities and previously called themselves Ansar Bayt al-Maqdis, before pledging allegiance to ISIL last year to become ISIL Sinai," the official said.
In addition, CBS News was told "terrorist groups are often focused on local grievances, which helps extremists present themselves as defenders of their communities and develop grassroots support."
The counterterrorism official went on to say, "It wouldn't be surprising if future attacks by the group feature a mix of targets to satisfy their focus on targeting the Egyptian government, while also heeding ISIL leadership calls to strike enemies of the so-called caliphate."
An ISIS video lauds its affiliate in Sinai for bringing down the plane, a claim that intelligence services are increasingly convinced is true.
An ISIS statement said the operation was retaliation for Russian airstrikes in Iraq and Syria.
Sources said Egypt's Sinai Peninsula is the wild west and a "hot bed for loose weapons." A counterterrorism official told CBS News, "the Sinai is well-known for illicit smuggling and ISIL Sinai has been able to acquire and use a range of weapons. As an ISIL affiliate, the group could also draw on the resources and expertise of other ISIL affiliates and supporters in the region."
The U.S. government has not confirmed reports that the cockpit voice recorder from the Russian passenger jet that crashed in Egypt reveals an explosion, but other new evidence is bolstering the theory that a bomb did tear through the plane.
An Egyptian official heading the investigation team looking into the crash said over the weekend that a noise can be heard on the recorder, about one second before the recording ends, but that it could not be definitively declared an explosion. Sources told CBS News last week that an explosion was believed to be audible.
On Monday, U.S. intelligence sources told CBS News that communications intercepts between ISIS suspects heighten the credibility of the group's claims to have downed the plane. Sources said there were varying amounts of detail in the chatter that make it "more likely than not" the terror group was involved.
A U.S. official said American agencies had intercepted chatter from ISIS members claiming they had an insider at the airport in Sharm el-Sheikh. The FBI has been invited by the Russians to go to the scene of the investigation.
The FBI has offered assistance to Russia and egypt and "stands ready to assist," FBI spokesman Joshua Campbell told CBS News.
CBS News correspondent Allen Pizzey reported the first of three teams of Russian inspectors was expected to arrive Monday to begin examining security at the airport in Sharm el-Sheikh, from which the Metrojet plane took off about 20 minutes before crashing into Sinai.