Counter-terrorism detectives are continuing to question four men, including one suspected of having links to Islamic State (Isis), on suspicion of plotting a terror attack on Britain.
The men, aged from 20 to 21, were arrested in raids across London on Tuesday to disrupt what investigators believe could have been the early stages of a significant plot.
It is believed that one of the men arrested was Tarik Hassane, 21. Neighbours of Hassane described hearing a loud commotion as police officers raided his family’s first-floor flat at 5am on Tuesday.
“There were three bangs – it woke me up. Police were there all day stripping the place,” said one neighbour.
Another said police were searching the flat until late in the evening, with officers replacing the front door, which had been taken off its hinges during the raid.
A Metropolitan police spokesman confirmed on Wednesday that the men remained in custody for further questioning.
The four were arrested “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism” in what the Met police commissioner, Sir Bernard Hogan-Howe, described as a “quite serious case”.
The seriousness of the investigation was underlined by the presence of specialist firearms officers at one address, which counter-terrorism officials said was due to a concern that the individual had access to weapons. Another of the suspects was Tasered by police.
Hogan-Howe told BBC London: “It is a quite serious case and it’s one of a series of arrests that we’ve had over the last few weeks, which taken together for me confirm that the drumbeat around terrorism has changed. It’s a more intense drumbeat. We’re having to be more interventionist and a lot of it is linked back to Syria and Iraq.”
Hassane, a medical student, wrote on social media 10 months ago that he was offered a place to study biomedicine at King’s College London. He instead decided to study in Sudan, starting a four-year course in 2013.
Writing on an Ask.fm page, Hassane suggested that he preferred to study in Sudan but said he planned to return to London during the holidays and to seek a placement at a hospital when he graduated.
Police searched his family home at Princess Alice House, near Ladbroke Grove in west London, but neighbours said he was rarely seen at the address.
“I’ve been living here for five years and I’ve never seen a young man there, just a woman,” said one resident in the same block of flats.
Nesreen Ahmed, 31, a mother of two little girls, said she was woken at around 4am on Tuesday. She said that all the neighbours went out on to the balcony, but were shouted at to get inside by armed police. “I was very scared because of my two children.”
Assefa and Genet Negash, who live two flats above, said the Hassanes were a lovely family and very good neighbours. “The mother and the daughter are lovely women,” she said.
It is alleged that one of the four arrested men has a connection to Isis, the Islamist extremist group that has captured swaths of territory in Iraq and Syria and has been responsible for the beheading of western hostages, including two Britons.
Foreign policy and security analysts have warned there could be a retaliation in the UK in response to British fighter planes joining the US and some Arab states in bombing raids on Isis in Iraq. In a video last week, an Isis member called for Muslims in Britain to rise up to cause terror in the country.
However, the arrests are not connected to the Isis member with a British accent shown in footage of the beheadings. Although the man was masked in the videos, the FBI claims he has been identified.
The four, who are being held at police stations in central London, will be questioned again later by counter-terrorism officers. Police can usually hold suspects for up to 24 hours before deciding whether to charge or release them, but suspects arrested under the Terrorism Act can in the most serious cases be held without charge for 14 days.
The raids followed a planned operation, with at least some of the men having been subject to monitoring by counter-terrorism investigators for some time.
After a meeting between SO15 and MI5 at an executive liaison group, the decision was made to disrupt any plot. Counter-terrorism investigators believed they had enough material to stage arrests.
The decision to disrupt a suspected plot is usually taken because it is believed there is too great a chance of an attack or because investigators believe they have enough evidence to test their suspicions through the courts.
However, some past high-profile terror arrests have been based on intelligence that turned out to be inaccurate, and have led to accusations that police and MI5 have ramped up the nature of possible plots.
The fear of a terrorist attack being carried out by people with connections to Syria or Iraq led the Joint Terrorism Analysis Centre (JTAC) to recommend raising the terrorist threat level in August to severe, its second-highest warning.
Announcing the decision to change the threat level, the home secretary, Theresa May, said it was “related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west”.
Counter-terrorism investigators in the police and MI5 have faced an increasing workload as they assess and investigate potential threats from people with connections to several hotspots around the world.
The threat from Isis extremists is the main source of concern, but there remains anxiety about those with connections to alleged violent extremists in Syria and al-Qaida-linked groups there, as well as al-Shabaab in Somalia, and also al-Qaida-inspired terrorists with links to Pakistan.
The security services assess how dangerous terror suspects are, but in the two instances in the modern era during which people were murdered – the July 2005 attacks in London and last year’s attack on the soldier Lee Rigby – the violent jihadis responsible had slipped through the net.
After Tuesday’s arrests the terrorist threat level remained unchanged. This means that JTAC, which is housed within MI5, has advised ministers that an attack is highly likely.
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