Counter-terrorism officials identified the large group, of which roughly half are teenagers, as highly likely to flee to the ISIS strongholds of Iraq and Syria.
The 28 strong gang, which includes five teenage girls, were thought to be planning a terror attack in Brighton using knives or a pistol.
But it transpired five people, including three brothers, already slipped the clutches of UK security and managed to travel to war-torn Syria, joining up with Jabhat al-Nusra, an affiliate of al-Qaeda.
A further three, including one girl, have court orders preventing them from leaving for Syria.
The radicalised gang only emerged as part of a wider review for the council into three brothers who left their family home to take up arms in the country ravaged by civil war.
Amer, 22, Abdullah, 18, and Jaffar Deghayes, 17, left for Syria in 2014, along with their childhood friend Ibrahim Kamara, 19.
All but Amer are thought to have been killed fighting, who is still believed to be in Syria.
A report by counterterrorism officials revealed they feared some of the radicalised group were planning a Lee Rigby style attack on the streets of Brighton, home to 273,000 people.
Speaking to The Sunday Times, a source said: “The first threat level that they were concerned about is that some of them would take a knife and kill people, a bit like what happened in the Lee Rigby attack”, Express reported.
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