The United States government is tracking as many as 300 Americans supposedly fighting with so-called Islamic State of Iraq and the Levant (ISIL) terrorist group, according to senior US officials. Washington is worried that radicalized foreign militants could become a risk to the US if they return to employ skills learned overseas to carry out attacks, anonymous US officials said, according to the Washington Times. “We know that there are several hundred American passport holders running around with ISIL in Syria or Iraq,” a senior US official said. “It’s hard to tell whether or not they’re in Syria or moved to Iraq.” Past reports have put the possible number of Americans who have flocked to ISIL at around 100. The US State Department did not respond to the Washington Times when asked to comment on Americans fighting in Iraq or Syria. The news of as many as 300 Americans fighting with IS comes one day after reports that a 33-year-old American, Douglas McAuthur McCain, was killed over the weekend in Syria while battling alongside ISIL terrorists. Family members confirmed his passing to NBC News, and senior US officials acknowledged that they were aware of the man’s death. McCain had expressed support for ISIL on his Twitter account. Other ISIL supporters have taken to social media to inspire anxiety among the US government. In recent weeks, photos were posted on Twitter showing the ISIL flag unfurled in front of the White House, and, in front of the Old Republic building on Chicago's Michigan Avenue, a message in Arabic was shown, reading, “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 photo near the White House. The FBI and the Department of Homeland Security issued last week a bulletin to local law enforcement agencies to be alert without identify a specific threat. The Obama administration believes any potential attack by American militants returning to the US would likely be carried out on a small scale, such as suicide bombings, and not a time-consuming long-range plan, according to White House spokesman Josh Earnest. Senior US officials said, according to the Washington Times, that intelligence agencies will put any Americans affiliated with ISIL on an appropriate watch list or no-fly list. Despite all the consternation in official Washington over such potential threats, observers note that ISIL has gained strength from the financial backing of United States’ allies in the Middle East, including Saudi Arabia, Kuwait, and Qatar.