A former Guantanamo Bay prisoner was among four Britons arrested on Tuesday in central England on suspected terrorism offences related to Syria, police said.
Moazzam Begg, 45, who was released without charge from Guantanamo Bay in 2005, was detained at his home in Birmingham on suspicion of attending a terrorist training camp and facilitating terrorism overseas.
Begg was held by the US government at Bagram detention centre in Afghanistan then Guantanamo Bay for nearly three years after being arrested in Pakistan in February 2002 suspected of being a member of al-Qaeda.
After his release, he founded Cageprisoners, a human rights organization that lobbies on behalf of those held at Guantanamo Bay.
"We can confirm that Moazzam Begg was arrested this morning," West Midlands police said in a statement. "This is an arrest, not a charge."
No comment on the arrest from Begg was immediately available.
Guantanamo Bay, a US military prison in Cuba, was established in 2002. President Barack Obama said in January that 2014 should be the year to finally close down the internationally condemned facility.
Its remaining 155 prisoners were rounded up overseas after the September 11, 2001, attacks on the United States and have been held without trial ever since.
The other three held in Britain on Tuesday - a 36-year-old man from Shirley and a 44-year-old woman and her 20-year old son from the Sparkhill area of Birmingham - were detained on suspicion of facilitating terrorism overseas.
"All four arrests are connected. They were pre-planned and intelligence-led," said the head of investigations for West Midlands Counter-Terrorism Unit, Detective Superintendent Shaun Edwards, in the statement.
Concern has been mounting in Britain over the number of UK nationals travelling to Syria to help rebels fighting President Bashar al-Assad. Police fear they may become radicalized by Islamists or attend terrorist training camps before returning to the UK where they could pose a security risk.
British police have already arrested 16 people on suspicion of terrorism offences related to Syria this year, some as young as 17, compared to 24 such arrests in all of 2013.
"A growing proportion of our casework now has some link to Syria, mostly concerning individuals from the UK who have travelled to fight there or who aspire to do so," Andrew Parker, director general of Britain's MI5 security service, said in a speech last October.