(CNN) The trail for the eighth Paris attacker, Salah Abdeslam , has gone cold, a senior European counterterrorism official told CNN late Thursday, who also said intelligence indicates ISIS wishes to strike the United Kingdom as a followup to its recent attack in France .

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.

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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.

Photos: World reacts to Paris attacks President Barack Obama, French President Francois Hollande, second from right, and Paris Mayor Anne Hidalgo arrive at the Bataclan, site of one of the Paris terrorists attacks, to pay their respects to the victims after Obama arrived in town for the COP21 climate change conference early on Monday, November 30, in Paris. Hide Caption 1 of 40 Photos: World reacts to Paris attacks The Eiffel Tower in Paris is illuminated in the French national colors on Monday, November 16. Displays of support for the French people were evident at landmarks around the globe after the deadly terrorist attacks in Paris on Friday, November 13. Hide Caption 2 of 40 Photos: World reacts to Paris attacks People hold hands as they observe a minute of silence in Lyon, France, on November 16, three days after the Paris attacks. A minute of silence was observed throughout the country in memory of the victims of the country's deadliest violence since World War II. Hide Caption 3 of 40 Photos: World reacts to Paris attacks French President Francois Hollande, center, flanked by French Prime Minister Manuel Valls, right, and French Education Minister Najat Vallaud-Belkacem, center left, stands among students during a minute of silence in the courtyard of the Sorbonne University in Paris on November 16. Hide Caption 4 of 40 Photos: World reacts to Paris attacks A large crowd gathers to lay flowers and candles in front of the Carillon restaurant in Paris on Sunday, November 15. Hide Caption 5 of 40 Photos: World reacts to Paris attacks A man sits next to candles lit as homage to the victims of the deadly attacks in Paris at a square in Rio de Janeiro on November 15. Hide Caption 6 of 40 Photos: World reacts to Paris attacks People light candles in tribute to the Paris victims on November 15 in Budapest, Hungary. Hide Caption 7 of 40 Photos: World reacts to Paris attacks People gather outside Notre Dame Cathedral in Paris on November 15 for a national service for the victims of the city's terror attacks. Hide Caption 8 of 40 Photos: World reacts to Paris attacks People write messages on the ground at Place de la Republique in Paris on November 15. Hide Caption 9 of 40 Photos: World reacts to Paris attacks People pray during a candlelight vigil for victims of the Paris attacks at a church in Islamabad, Pakistan, on November 15. Hide Caption 10 of 40 Photos: World reacts to Paris attacks French golfer Gregory Bourdy passes a peace symbol for the Paris victims during the BMW Shanghai Masters tournament November 15 in Shanghai, China. Hide Caption 11 of 40 Photos: World reacts to Paris attacks A man offers a prayer in memory of victims of the Paris attacks at the French Embassy in Tokyo on November 15. Hide Caption 12 of 40 Photos: World reacts to Paris attacks A woman holds a candle atop a miniature replica of the Eiffel Tower during a candlelight vigil Saturday, November 14, in Vancouver, British Columbia. Hide Caption 13 of 40 Photos: World reacts to Paris attacks Front pages of Japanese newspapers in Tokyo show coverage and photos of the Paris attacks on November 14. Hide Caption 14 of 40 Photos: World reacts to Paris attacks An electronic billboard on a canal in Milan, Italy reads, in French, "I'm Paris," on November 14. Hide Caption 15 of 40 Photos: World reacts to Paris attacks The Eiffel Tower stands dark as a mourning gesture on November 14, in Paris. More than 125 people were killed in a series of coordinated attacks in Paris on Friday. People around the world reacted in horror to the deadly terrorist assaults. Hide Caption 16 of 40 Photos: World reacts to Paris attacks Lithuanians hold a candlelight vigil in front of the French Embassy in Vilnius, Lithuania, on November 14. Hide Caption 17 of 40 Photos: World reacts to Paris attacks Thousands gather in London's Trafalgar Square for a candlelit vigil on November 14 to honor the victims of the Paris attacks. Hide Caption 18 of 40 Photos: World reacts to Paris attacks A woman lights candles at a memorial near the Bataclan theater in Paris on November 14. Hide Caption 19 of 40 Photos: World reacts to Paris attacks A man places a candle in front of Le Carillon cafe in Paris on November 14. Hide Caption 20 of 40 Photos: World reacts to Paris attacks A woman holds a French flag during a gathering in Stockholm, Sweden, on November 14. Hide Caption 21 of 40 Photos: World reacts to Paris attacks Nancy Acevedo prays for France during the opening prayer for the Sunshine Summit being held at Rosen Shingle Creek in Orlando, Florida on November 14. Hide Caption 22 of 40 Photos: World reacts to Paris attacks French soldiers of the United Nations' interim forces in Lebanon observe the national flag at half-staff at the contingent headquarters in the village of Deir Kifa on November 14. Hide Caption 23 of 40 Photos: World reacts to Paris attacks A couple surveys the signature sails of the Sydney Opera House lit in the colors of the French flag in Sydney on November 14. Hide Caption 24 of 40 Photos: World reacts to Paris attacks A woman places flowers in front of the French Consulate in St. Petersburg, Russia, on November 14. Hide Caption 25 of 40 Photos: World reacts to Paris attacks Candles are lit in Hong Kong on November 14 to remember the scores who died in France. Hide Caption 26 of 40 Photos: World reacts to Paris attacks A woman lights a candle outside the French Consulate in Barcelona, Spain, on November 14. Hide Caption 27 of 40 Photos: World reacts to Paris attacks Britain's Prince Charles expresses solidarity with France at a birthday barbecue in his honor near Perth, Australia, on November 14. Hide Caption 28 of 40 Photos: World reacts to Paris attacks The French national flag flutters at half-staff on November 14 at its embassy in Beijing. Hide Caption 29 of 40 Photos: World reacts to Paris attacks Dutch Prime Minister Mark Rutte after a speech on November 14 in The Hague following the attacks. Hide Caption 30 of 40 Photos: World reacts to Paris attacks Japanese Prime Minister Shinzo Abe becomes emotional after his speech on the French attacks during the opening ceremony of a Japanese garden in Istanbul, Turkey, on November 14. Hide Caption 31 of 40 Photos: World reacts to Paris attacks A woman mourns outside Le Carillon bar in the 10th district of Paris on November 14. The attackers ruthlessly sought out soft targets where people were getting their weekends underway. Hide Caption 32 of 40 Photos: World reacts to Paris attacks People lay flowers outside the French Embassy in Moscow on November 14. Hide Caption 33 of 40 Photos: World reacts to Paris attacks Mourners gather outside Le Carillon bar in the 10th district of Paris on November 14. "We were listening to music when we heard what we thought were the sounds of firecrackers," a doctor from a nearby hospital who was drinking in the bar with colleagues told Le Monde. "A few moments later, it was a scene straight out of a war. Blood everywhere." Hide Caption 34 of 40 Photos: World reacts to Paris attacks People attend a vigil outside the French Consulate in Montreal. Prime Minister Justin Trudeau offered "all of Canada's support" to France on Friday, November 13, in the wake of the attacks. Hide Caption 35 of 40 Photos: World reacts to Paris attacks Police show a heightened presence in Times Square in New York on November 13, following the terrorist attacks in Paris. Hide Caption 36 of 40 Photos: World reacts to Paris attacks People light candles at a vigil outside the French Consulate in Montreal on November 13. Hide Caption 37 of 40 Photos: World reacts to Paris attacks University of Nevada, Las Vegas, fans observe a moment of silence for the victims of the terrorist attacks in Paris before a basketball game November 13. Hide Caption 38 of 40 Photos: World reacts to Paris attacks The house lights are shut off and scoreboard dark as Boston Celtics players pause for a moment of silence for the Paris victims before an NBA basketball game against the Atlanta Hawks in Boston on November 13. Hide Caption 39 of 40 Photos: World reacts to Paris attacks People light candles at a vigil outside the French Consulate in Montreal on November 13. Hide Caption 40 of 40

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.

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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.