SHARM EL-SHEIKH, Egypt -- 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.
Russia has requested support from the FBI in the investigation, and the U.S. is "working" with the Russians, but American officials said earlier that Washington had not "shared specific information" with Moscow yet.
The leading theory, according to U.S. intelligence, has been since last week that a bomb caused the Airbus A321 to break up over Egypt's restive Sinai Peninsula.
CBS News correspondent Allen Pizzey says 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.
Inspectors will likely look not only at whether security measures have been improved, but also how they may have been breached.
With the search for wreckage and bodies almost completed Russian emergency workers held a memorial ceremony at the crash site, laying roses on one of the plane's wings.
A full analysis of the evidence they've found will take time, and Russian investigators have asked the FBI for assistance.
Fourteen year-old victim Alisa Vitalieva was buried over the weekend, mourned by her grandparents among many others. She was travelling with her mother Irina, whose body has not yet been identified.
Identifying the culprits may be easier.
An ISIS video lauds its affiliate in Sinai for bringing down the plane; a claim that intelligence services are now all but one hundred percent sure is true.
An ISIS statement said the operation was retaliation for Russian airstrikes in Iraq and Syria.
Russia has begun a monumental airlift to bring 79,000 of its citizens out of Egypt, but President Vladimir Putin won't let it go at that if ISIS is guilty.
CBS News senior security contributor and former CIA deputy director Michael Morell says Putin's reaction would "be to go after ISIS in a very big way to show them the cost of doing this to his country."
A memorial service in St. Petersburg's cathedral over the weekend underscored the price already paid.
The bell tolled 224 times, one for each victim of the crash.
About 2,224 miles to the south, underscoring the theory that someone sneaked a bomb on board the plane, workers at Sharm el-Sheikh airport said they had been questioned by Egyptian security officials, and planes sitting on the tarmac overnight had armed guards.