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 .
NEW : The two suspects are British nationals , police sources say . The man and woman , both 26 , were arrested after arriving on a flight . Officers are also searching two homes in east London . UK Foreign Office says it takes any claims of Britons fighting in Syria very seriously .