Washington (CNN)A U.S. official told CNN it was the "specificity" in the chatter surrounding the crash of the Russian jet in Egypt and "the specific nature of the discussion" being monitored that drew the attention of the U.S. intelligence community.
How chatter helped drive U.S. suspicion of bomb plot
However, the first official stressed, until the Russians and Egyptians share the physical pieces of the wreck, the public may never know what caused the crash. The official said it is still possible this was some sort of structural accident and that no official conclusion has been reached, an idea echoed by other U.S. officials who cautioned that this was not a final intelligence analysis.
The communications included people associated with an ISIS affiliate in the Sinai Peninsula, where the jet came down, talking about the bomb's origin and bragging about the crash, a counterterrorism official said.
Another U.S. official told CNN that message traffic involving the ISIS affiliate in the Sinai was the first key indicator after the crash that it might have been a terrorist attack.
The counterterrorism official warned that sometimes the chatter is empty boasting to impress others or even jihadi attempts to throw off intelligence agencies with misinformation because they suspect their discussions are being monitored, though another official said it did not appear to be false bragging but rather a discussion of the crash that had to be taken seriously.
And one reason some U.S. counterterrorism officials remain very cautious is that a lot of the chatter comes from open sources, like online forums, social media and other Internet resources.
What gives it more credence, though, is that the messages come from users who have seemed to know what they're talking about in the past, according to counterterrorism officials who spoke with CNN. They have enough information that it gives analysts a measure of confidence that it could be real.
U.S. officials told CNN that the chatter with the specificity that caught the U.S. intelligence community's attention was only picked up after the plane went down and that the U.S. did not have any specific knowledge of a potential plot before it happened.
However, one of the U.S. officials who was briefed on the investigation said there had been chatter about bomb capabilities prior to the crash. Another official told CNN on Wednesday that prior to the incident, "there had been additional activity in Sinai that had caught our attention," though the official did not explain what that activity was.
Other counterterrorism officials described this prior information as intercepts they had recovered referring to the growing capabilities of militants in the Sinai generally, rather than about this event.
Open-source information is a major new trend in the way U.S. intelligence works, according to counterterrorism officials.
It's a method that the head of the National Counterterrorism Center calls a "paradigm shift" for the intelligence community.
"I can remember previous instances in which we've looked at particular terrorism incidents or events in which we have focused almost exclusively on clandestinely collected intelligence, either signals or human intelligence collected by one of our intelligence community partners. That is no longer the primary way of doing business as we think about the plots," said NCTC Director Nick Rasmussen in an interview with CNN terrorism analyst Paul Cruickshank.
Edward Snowden, the former NSA employee, leaked reams of detail about how the U.S. has gathered information online using clandestine methods.
But terrorists are also using more social media and other publicly available forums to hide, a counterintuitive idea that boils down to creating ever bigger haystacks in which they can slip their needles.
But for intelligence analysts, it makes it hard to discern who's just blowing smoke and who actually knows what they're talking about.
The chairman of the Senate intelligence committee cautioned Thursday that the evidence about the crash in the Sinai is circumstantial.
"I think any catastrophic event needs to be investigated before you reach any firm conclusions," Sen. Richard Burr of North Carolina told reporters as he walked to the Senate floor for a vote on Thursday. "It's a part of the world where anything is possible, including mechanical failure."