Three hundred Swedish citizens travelled to Syria and Iraq to join the Islamic State, according to Swedish Security Service (SAPO) on Tuesday. SAPO's General Director Anders Thornberg revealed that 100 fighters returned to the country posing serious threat against the security. He added that a number of those returnees plan to execute terrorist attack in the country. The security official said that the arrest of those jihadists is a top priority for the Swedish authorities. However, the main challenge in this regard is the increasing refugees among whom the terrorists are hiding, he added