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.
GETTY People relax on deckchairs on Brighton beach which could have been among the jihadi group's target
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.”
The source added: “It’s obviously a serious concern that there are 28 young people brainwashed to the point that they want to join jihadist groups or consider plotting against innocent people here at home.
“The concern has not abated; there’s a lot of extremism-related problems lurking that the police nd social services are attempting to deal with.”
ALAMY Brighton group thought to be planning attack using guns and knives
Names of the 28 had appeared on lists of people deemed to be at risk of travelling to Syria, and were known to counter-terrorism police.
Roughly 60 per cent of the crew are Islamic converts, and at least one girl had previously been referred to the government’s counter-radicalisation programme, Channel.