European security agencies have had no trace of Abdeslam since he was dropped off by a friend in the Brussels district of Schaerbeek on November 14, the day after the attacks. The official said late Thursday investigators now believe he developed cold feet about blowing himself up in Paris.
"He was freaking out, he was scared when he called his friends in Brussels to come and pick him up that night from Paris," the official told CNN.
The official said he believed ISIS might not welcome him back into the fold, even if he was able to reach Syria.
Intel: United Kingdom is next ISIS target
Meanwhile, intelligence obtained by European security agencies indicates that ISIS is aiming to attack the United Kingdom in a follow-up to its Paris operation, the official said.
The intelligence suggests British ISIS fighters have been tasked by senior ISIS operatives in the so-called caliphate to return home and carry out an attack.
It's not clear how imminent the threat is nor the specific location. However, the official said the concerns have been compounded by the vote in the House of Commons on Wednesday to authorize British airstrikes against ISIS in Syria.
The official did not say whether one factor in the intelligence warning was the arrest of a British ISIS operative linked to "Jihadi John" in Turkey in November.
Aine Lesley Davis, a British ISIS operative, was arrested in Istanbul the same day as the Paris attacks as he planned to travel to Europe to deliver orders on planned terror attacks, a Turkish official told CNN last month. The information was first reported by the leading Turkish newspaper Hurriyet, whose account a Turkish official confirmed to CNN.
Davis was arrested after Turkish intelligence worked with Britain's MI6 service to monitor the movements of a messenger linked to Jihadi John inside the Syrian city of Raqqa, Hurriyet said.
Intelligence on possible locations in Syria for Davis' colleague, ISIS militant Jihadi John, were also shared by Turkish National Intelligence to the CIA and MI6. The airstrikes in Raqqa that U.S. officials say killed Jihadi John
, real name Mohammed Emwazi, on November 12 followed hours later, the paper said.
A Tunisian connection
The senior official also told CNN that intelligence suggests one of the most senior figures in ISIS's external operations unit is a Tunisian ISIS operative, who is believed to have been one of the brains behind the Paris attacks and continues to be involved in plans to attack the West.
They said Western intelligence agencies are aware of his identity and that the Tunisian was working with several French ISIS fighters in and around Raqqa, the de facto ISIS capital in Syria.
The official said it was possible the external operations cell reports up to Abu Mohammed al-Adnani, the top ISIS leader in Syria, who has repeatedly issued fatwas calling for attacks against Western countries involved in airstrikes against ISIS.
CNN's Barbara Starr has previously reported Adnani is the head of ISIS's external operations, according to U.S. officials.
More on the Paris attacks
The official said one of the suspected brains behind the Paris attacks was Fabien Clain, a veteran French ISIS jihadi who claimed responsibility for Paris in an ISIS audiotape the day after the attacks.
Intelligence suggests Clain and Abdelhamid Abaaoud were working in tandem to recruit operatives for a string of attacks against France this past year. Abaaoud was not the overall mastermind of the Paris attacks but rather a ringleader, said the official.
Even so, European intelligence agencies are surprised ISIS decided to sacrifice him in launching the attacks instead of keeping him available for future operations. The official said it was still unclear when Abaaoud returned to Europe from Syria.
The official provided new details on the intelligence that led French police to a residence in St. Denis on November 18 where Abdelhamid Abaaoud and several co-conspirators were hiding and plotting a new wave of attacks.
The key intelligence came from a female "walk-in" who alerted French police that Abaaoud was in Paris with his female cousin Hasna Aït Boulahcen. The French initially discounted the information, but then decided to trail Boulahcen, which led them to the safe house.
Why Brussels went on full alert
The official said the decision to raise the terrorism alert to the maximum level in Brussels a week after the Paris attacks was not based on concrete information on a specific plot but was made because of the broader concern about the potential threat posed by Belgian-French network behind the Paris attack, given several plotters remained at large.
The alert level was subsequently lowered.
The official said Belgian security services have located the Belgian residence used to make the suicide vests, which the plotters had made significant attempts to clean of all traces of explosives before launching the Paris attacks. The official said the bomb-maker who made the vests is still at large.
The official said that Ahmet Dahmani, a Belgian citizen who was arrested in Antalya, Turkey, on November 21, was in touch with and close to Salah Abdeslam and is suspected of possibly helping with the attacks. CNN previously reported that Dahmani and Abdeslam traveled together from Italy to Greece and back by ferry in early August.
Dahmani is believed to have flown to Turkey from Amsterdam the day after the attacks, where according to Turkish officials, he was met by ISIS escorts in a luxury hotel who had been tasked with bringing him across the border to Syria.
The official described the Belgian-French network behind the Paris attacks as an informal network based on friends who had been part of gangs and spent time in prison together growing up.