NEW : The two suspects are British nationals , police sources say . A third suspect is still at large and may be armed with a knife, according to the source who spoke on condition of anonymity because he was not authorized to speak about an ongoing investigation. One line of inquiry being followed by investigators , the police sources said , is whether the two suspects were involved in an incident in July in which Sunday Times photographer John Cantlie said he was held at gunpoint by rebels in Syria . Officers are also searching two homes in east London . UK Foreign Office says it takes any claims of Britons fighting in Syria very seriously .
NEW : The two suspects are British nationals , police sources say . A third suspect is still at large and may be armed with a knife, according to the source who spoke on condition of anonymity because he was not authorized to speak about an ongoing investigation. Officers are also searching two homes in east London . UK Foreign Office says it takes any claims of Britons fighting in Syria very seriously .