FBI tracking some Americans who fought with islamic State, then returned home

White House officials on Monday acknowledged for the first time that Americans who’ve fought alongside the Islamic State terrorist group in Syria and Iraq have returned to the U.S., raising concerns about an attack on the homeland as President Obama prepares to urge the United Nations to confront the threat overseas.

Counterterrorism experts believe at least 100 Americans have traveled to the Middle East to join the terrorist group, and a senior administration official said the total includes “those who’ve gone, those who’ve tried to go, some who’ve come back and are under active [investigation] — the FBI is looking at them.”

“It is … believed that some 40 of those who left this country to join up with [the Islamic State] have now returned to our country,” said Rep. Timothy H. Bishop, New York Democrat. He said the 40 individuals “are under FBI attention and surveillance. So they are known and being tracked by the FBI.”

The development heightens one of the administration’s gravest concerns about the terrorist group that Mr. Obama once referred to as a “JV” organization: that battle-hardened jihadis holding U.S. passports could slip into the U.S. undetected and plan attacks virtually anywhere.