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. Those concerns were heightened by the disclosure Tuesday that a California man was killed fighting alongside militants with the group, also known as ISIS.

The U.S. government is doing its best to keep track of the foreign fighters, who have been shifting back and forth between Iraq and Syria, according to a senior U.S. official.

“We know that there are several hundred American passport holders running around with ISIS in Syria or Iraq,” the official said, offering a figure well above widespread reports of about 100 such fighters. “It’s hard to tell whether or not they’re in Syria or moved to Iraq.”

The State Department did not respond to a request for the number of Americans traveling in Iraq and Syria.

Supporters of the Islamic State group have worked to cultivate anxiety in the United States over the threat they might pose domestically.

Among photographs posted to Twitter in recent weeks is one that shows the Islamic State flag unfurled in front of the White House at night. Another photo taken in front of a high-rise building in Chicago features the message: “We are in your state. We are in your cities. We are in your streets. You are our goals anywhere.”

The Secret Service is investigating the incident in front of the White House, and the FBI and Department of Homeland Security last week issued bulletins to local law enforcement agencies urging them to be alert but not identifying any specific threat.

The militant group’s supporters have also pursued a more subtle propaganda campaign on social media against Mr. Obama’s policies. One post includes a photo of Mr. Obama on the golf course with insets of U.S. soldiers in combat and American flag-draped coffins. The caption reads: “O American soldiers! Obama sends you to death and he just goes to play golf.”

In even some Western countries, the tactics appear to be gaining traction. London-based polling firm ICM Research conducted a survey in Britain, France and Germany to test whether people were aware of the Islamic State and in favor of the group’s radical actions.

At least 15 percent of the roughly 1,000 French citizens polled responded favorably to the group’s terror tactics, according to Russia Today. ICM conducted the poll on behalf of Russian news agency Rossiya Segodnya.

The poll was conducted Aug. 19, the same day a videotape surfaced showing a member of the Islamic State beheading American journalist James Foley. It was unclear whether the poll was prompted by Foley’s killing, which his executioners said was done to send a message to President Obama and the U.S. military.

“You’re no longer fighting an insurgency. We are an Islamic army and a state that has been accepted by large number of Muslims worldwide,” Mr. Foley’s executioner said. “So effectively, any aggression towards the Islamic State is aggression towards Muslims from all walks of life who has accepted the Islamic caliphate as their leadership. So any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people.”

The threat of radical jihadists returning to the United States is “a new hazard” for the Department of Homeland Security, said retired Army Maj. Mike Lyons, a senior fellow with the Truman National Security Project and a CBS Radio News analyst. As a result, the department must plan for a future where the jihadists leave their Middle East battlefield and trickle back to America, he said.

“If these people have been identified, there needs to be a discussion with regard to how and when they are allowed back in the U.S.,” he said.

Heightening concerns about Americans joining the Islamic State group were reports Tuesday that a California man who fought side by side with militants was found dead on the battlefield.

U.S. officials confirmed Tuesday that the body of Douglas McAuthur McCain, 33, was found after a brutal battle. He was identified based on the U.S. passport he had in his pocket.

National Security Council spokeswoman Caitlin Hayden said officials were “aware” of McCain’s presence in Syria and acknowledged that the White House had been tracking his whereabouts prior to his death.

“We continue to use every tool we possess to disrupt and dissuade individuals from traveling abroad for violent jihad and to track and engage those who return,” she said.

CNN spoke to McCain’s uncle, Ken McCain, who said his nephew converted from Christianity to Islam several years ago, became a jihadi and left his family “devastated.” Douglas McCain often praised the Islamic State on his Twitter account.

The discovery of an American Muslim convert in combat alongside foreign fighters is consistent with the unease of U.S. officials, who are concerned they will take what they learned from the battlefield, return to the homeland and perpetrate attacks on Americans.

Obama administration officials believe the threat capability of the returning jihadists is limited to small attacks. The foreign fighters are expected to lack the conspiracy planning and organization necessary to commit a larger attack like the Sept. 11, 2001, terrorist attack, White House press secretary Josh Earnest said.

But Maj. Lyons suggested the foreign fighters overall present a greater long-term danger to the United States than al-Qaeda.

“Unlike the 9/11 terrorists, who worked as a team and [carried out] a plan over the course of years, individuals returning from Syria are likely a greater risk to be either lone wolf suicide bomber-type attackers or organizers/recruiters for cells who will conduct a more complex attack than a suicide bomb,” he said.

Senior U.S. officials said that it is likely that intelligence agencies will place those Americans on an appropriate watch list or a no-fly list.

“I know that law enforcement agencies in Homeland Security are mindful of some Americans who have become radicalized, and some have taken up with [the Islamic State],” the official said.

The Department of Homeland Security did not respond to a request for clarification on its methods for tracking Americans suspected of involvement with the Islamic State group or for deterring the potential threat they pose to the United States.

Not everyone in Washington sees such a small sliver of the radicalized American population as a threat to the U.S. homeland.

Aaron Miller, a national security analyst and the vice president for new initiatives at Washington think tank the Woodrow Wilson International Center for Scholars, said members of the Islamic State group are projected to range between 10,000 to 15,000 — a figure that does not pose a serious threat to the United States and should not spark a scare campaign.

“We love scaring ourselves,” said Mr. Miller, who has served as adviser to Republican and Democratic secretaries of state. “I mean, we’ve become masters at it. We did it during the Cold War and we did it in the wake of 9/11. And while our response to Afghanistan was extremely appropriate, look what we did in Iraq.”

U.S. allies, however, are also taking seriously the threat posed by Islamic State foreign fighters and showing increasing concern about Westerners who have joined the militants and hold passports that would allow them to travel to their countries of origin to carry out terrorist attacks.

The British government issued an urgent appeal to the public Tuesday to help police identify “aspiring terrorists” who may be preparing to strike the West as officials work to confirm whether the man seen in the video executing Foley was a British citizen.

“They may be about to travel abroad, have just returned or be showing signs of becoming radicalized,” said Mark Rowley, a top British counterterrorism official. “We appeal to the public to help identify for us aspiring terrorists. High-priority operations, especially against those involved in attack planning or on the cusp, have increased greatly.”

• Dave Boyer and Philip Swarts contributed to this report.

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