The U.S. government is tracking and gathering intelligence on as many as 300 Americans who are fighting side by side with the Islamic State group in Iraq and Syria and are poised to become a major threat to the homeland, according to senior U.S. officials. Officials say concern is widespread in Washington that radicalized foreign fighters could return to the homeland and commit terrorist attacks with skills acquired overseas, according to officials who spoke on condition of anonymity due to the sensitivity of the information.