Sky News home affairs correspondent Mark White said that, as a rule, while many people are initially arrested after a terrorist attack - including family members of the suspects - they may be released after questioning and investigations.
TERROR related arrests have surged to a new record high, with suspects held at a rate of more than one every day as security services confront an unprecedented threat.
In 12 months leading to March 2016 terrorism related arrests were up by 18 per cent to 304, and was the high number in any financial year since data collection started in September 2001, after the 9/11 attacks on the Twin Towers and the Pentagon.
That represents a 68 per cent increase over the year before.
Deputy Assistant Commissioner Neil Basu, the senior national co-ordinator for counter-terrorism policing, said: "We would describe this as a shift not a spike".
The UK has suffered a torrid year of terror attacks, with five killed in the Westminster Bridge attack, 22 dying in the Manchester Arena bombing and eight killed in the London Bridge atrocity.
By the end of June this year, 204 people in the United Kingdom were in custody for terrorism-related offences in the country.
Of those arrested, 50% were eventually released without charge, while 105 people were charged with terror-related offences. The Home Office said 91% of those prisoners held extreme Islamist views, while a further 5% had far-right ideologies.
Later that month, a man from south Wales drove a van into a crowd near Finsbury Park Mosque, killing one. In the next 17 weeks, however, there were the four attacks in Manchester and London, while authorities thwarted six others. "The tempo is increasing", UK Security Minister Ben Wallace told the Independent.