Police are continuing to question a former Guantanamo Bay detainee who has been arrested on suspicion of Syria-related terrorism offences.
Moazzam Begg, 45, from Hall Green, Birmingham, is suspected of attending a terrorist training camp and facilitating terrorism overseas, West Midlands Police said.
He was arrested yesterday morning along with a 36-year-old man from Shirley, in Solihull, and a 44-year-old woman and her 20-year-old son, both from Sparkhill, in Birmingham, who were held on suspicion of facilitating terrorism overseas.
Officers from the West Midlands Counter Terrorism Unit spent the day searching searching their three home addresses, and vehicles and electronic equipment were also removed for forensic analysis.
Detective Superintendent Shaun Edwards, head of investigations for West Midlands Counter Terrorism Unit, said all four arrests were connected.
Mr Begg was held in the US-run military prison at Guantanamo Bay in Cuba for nearly three years.
Non-governmental organisation Cage, which campaigns for the rights of people detained during counter-terrorism operations and for whom Mr Begg is an outreach director, said it was "outraged" by his arrest.
Asim Qureshi, research director of Cage, said: "Cage calls on all defenders of civil liberties and the rule of law to stand up and protest against the serious curtailment of yet another victim."
He added: "We fully support our colleague and see his arrest as politically motivated and as part of a campaign to criminalise legitimate activism."
Originally from Birmingham, Mr Begg moved to Afghanistan with his family in 2001 before taking them to Pakistan in 2002 when the war began.
He was detained in Islamabad, Pakistan, as an "enemy combatant" in January 2002 and was taken to the Bagram internment centre for about a year before being transferred to Guantanamo.
He was released along with three others in January 2005 and was allowed to return to the UK where he was arrested by the police before being released without charge.
Mr Begg has always maintained that he was only involved in charity business and that he has never been involved in any kind of terrorist activity.
He wrote about his travels to Syria in a publicly-available blog entry dated December 24, 2013 on the Cage website, in which he also described having his passport confiscated to prevent further travel.
In July 2012, Mr Begg wrote, he visited Syria and met former prisoners who had been held by the Assad regime.
In a second visit made in December 2012, he met current and former prisoners and also visited refugee camps.
As well as witnessing British aid being brought in, he claimed he also met British fighters.
Around 250 British-based extremists who went to train and fight in Syria have returned to the UK, it has emerged, more than half of the 400 Britons who have gone to Syria over the past two years.