A British human rights group expressed "outrage" today after one of its high-profile directors, former Guantanamo Bay detainee Moazzam Begg, was arrested in London on suspicion of terrorist activity in Syria.
"We are disgusted that Moazzam Begg is being re-traumatized with the same guilt by association accusations that resulted in his unlawful incarceration in Guantanamo Bay. We fully support our colleague and see his arrest as politically motivated and as part of a campaign to criminalize legitimate activism," Asim Qureshi, a senior official at CAGE, said in a statement.
Begg was arrested by British authorities with three other suspects based on unspecified intelligence, police said. None have been charged with a crime.
Begg has leveraged the country's libel laws against critics who have accused him of being a terrorist or sympathizer to al Qaeda, leaving many hesitant to comment publicly on his arrest for suspicion of attending a terrorist training camp in Syria. But security sources in London told ABC News that the arrest of Begg -- who CAGE said had suffered after three years spent in American terrorist prisons in Afghanistan and in Cuba -- was a gamble for the UK government because of his outspokenness in alleging past U.S. and British persecution. Sources speculated that it is likely the police have built a convincing case or they wouldn't have risked public blowback or embarrassment.
Back in 2002 Begg was arrested in Pakistan for suspected activities as a foreign fighter in Afghanistan and transferred to Guantanamo Bay in 2004, but was never charged or convicted for any terrorism-related crimes. He was released from Guantanamo in 2005 in a deal between U.S. and British security officials. After his release, Begg wrote a book in which he claimed he was shackled and beaten while in detention.
"My thought is that the Brits' suspicions about him have never left them," Robin Simcox, a terrorism analyst at the Henry Jackson Society in London, told ABC News on Tuesday after the arrest. "Blowback could happen. But from the state's point of view, you've got to enforce the law... To the police, Begg has been a character of interest for a long time."
Begg claimed he was abused at the American prison at Bagram Airfield in Afghanistan, where he was held for a year before transfer to Guantanamo. He has made no secret of his travels last year to Syria, which CAGE said involved "exposing British complicity in rendition and torture."
Some human rights groups did not immediately rush to defend Begg after his arrest Tuesday. The American Civil Liberties Union, which has been involved in the defense of other Guantanamo detainees, declined to comment when contacted by ABC News.
The war pitting many Syrians and foreign fighters -- often led by al Qaeda affiliate Jabhat al-Nusra and other ideological allies -- against the regime of Bashar al-Assad has been a major worry for both American and European security officials, who have Syria at the top of their counter-terrorism agenda.
U.S. Homeland Security Secretary Jeh Johnson recently returned from the European Union G-6 conference in Poland, where his counterparts repeatedly raised concerns over foreign fighters in Syria.
Intelligence officials on both sides of the Atlantic say that the ease of traveling to Syria means they cannot identify or track as many as half of those who have gone there to fight against Assad -- and in most cases as Islamist jihadis.
Officials have said they fear those trained in Syria or gaining battlefield experience could return to Europe or the U.S and conduct terror attacks there. Europeans are as worrisome as U.S persons with "clean" passports fighting in Syria because of their home countries' visa waiver status in traveling to the U.S., which carries less scrutiny.
"Assuming the evidence is solid, I'm not surprised at all that Begg might be involved in foreign fighter facilitation," Clint Watts, a senior fellow at the Homeland Security Policy Institute and former FBI agent, told ABC News.
Begg, who is active on Twitter, recently tweeted: "You may not want to hear it but it sounds like al Qaeda in Syria has most respect on the ground according to experts." Last week, he also tweeted a link to Watts' analysis on the evolving nature of al Qaeda networks.
U.S. Director of National Intelligence James Clapper has estimated there have been up to 7,500 foreign fighters there. An estimate by Washington Institute for Near-East Policy analyst Aaron Zelin put that number at more than 11,000.
Many have been U.S. persons, who may be citizens or immigrants. Sources told ABC News more than 50 have returned to the U.S. since early fall after fighting in Syria. Many of those who have returned are under various kinds of surveillance or investigation, FBI Director James Comey recently told reporters.
"The London Metropolitan Police and [domestic security service] MI-5 also have expressed this. Because the numbers are so great going to Syria, you only need a small number of those guys radicalized and you've got a big problem," said Simcox.
ABC News' Jean-Nicolas Fievet contributed to this report.