"We are not immune to an attack from a home-grown terrorist, but the probabilities and sustainability of such an act are very low," said DHS spokesman Michael Keegan.
That assessment came just hours after FBI Director Robert Mueller said his agency is "particularly concerned" that young men living in the United States could be recruited to attack the very country they call home.
"Since September 11th, we have learned of young men from communities in the United States, radicalized and recruited here to travel to [war-torn] countries such as Afghanistan or Iraq, Yemen or Somalia. They may be recruited to participate in the fighting, or, in the extreme case, to become suicide bombers," Mueller said during a discussion at the Council on Foreign Relations in Washington. "It raises the question of whether these young men will one day come home, and, if so, what they might undertake here."
Mueller called those cases "a perversion of the immigrant story."
"The parents of many of these young men risked everything to come to America, to provide their children with a brighter, more stable future," he said. "For these parents to leave a war-torn country only to find that their children have been convinced to return to that way of life is heartbreaking."
Mueller specifically referred to the case of a Minneapolis man -- "radicalized in his hometown" -- who blew himself up in Somalia last October. Mueller called him "the first U.S. citizen to carry out a terrorist suicide bombing." Authorities have identified the man as 27-year-old Shirwa Ahmed.
DHS and FBI officials both said they are not aware of any suicide attacks by U.S. citizens since Ahmed's attack several months ago.
Ahmed was part of a group of Somali-Americans who went missing from the Minneapolis area late last year. Authorities are still trying to track the rest of them down.
Meanwhile, Keegan, the DHS spokesman, said the American immigrant story is one reason the United States is less vulnerable to home-grown terrorism than other countries.
"People come to the United States to be part of something special, to practice their beliefs without worries of persecutions based on their religious faith, political views or personal life styles," he said. "When you give citizens the opportunity to live in an environment that promotes personal growth and happiness, you're essentially promoting the wellness and safety of an entire nation rather than a community of homegrown terrorist cells."
In March 2004, a series of coordinated bombings on the train system in Madrid, Spain, killed more than 190 people and wounded hundreds more. The next year, a team of suicide bombers blew themselves up on the London transit system, killing 52 passengers and themselves.