WASHINGTON (Reuters) - An Al-Qaeda-affiliated group in Somalia, al Shabaab, has recruited more than 40 Muslim Americans to its battle in the war-ravaged country and at least 15 have been killed, a congressional report said on Wednesday. US officials have become increasingly worried about the group, particularly after capturing an al Shabaab commander who had allegedly been a liaison with Qaeda in the Arabian Peninsula, an active Yemeni group. So far, al Shabaab has conducted only limited attacks outside of Somalia, notably the twin bombings in Uganda. Republican Peter King of New York, chairman of the House of Representatives Homeland Security Committee, has been holding a series of hearings to probe concerns about Muslim Americans becoming radicalized and joining militant groups. A report by his staff found that more than 40 Muslim Americans and 20 Canadians have been recruited to al Shabaab and at least 15 Americans were killed in fighting, including three suicide bombers. "Senior US counterterror officials have told the committee they are very concerned about individuals they have not identified who have fallen in with al-Shabaab during trips to Somalia, who could return to the US undetected," King said during a hearing he convened on al Shabaab. Of the more than 40 Americans who have joined the cause, as many as 21 are believed to still be at large and unaccounted for, according to the staff report. The US Justice Department has charged several people from the Somali community in Minnesota for allegedly going to Somalia to fight, as well as individuals who were accused of trying to help al Shabaab or those going to fight there. Anders Folk, a former federal prosecutor from Minnesota who handled some of those cases, said that while there are doubts about the ability of al Shabaab to strike the United States, it is hard to predict whether they might do so one day. "The difficulty is we don't know when they are going to cross the line from aspiration to operation and the reality is ... that cannot be predicted with any degree of certainty," he told the committee. Some Democrats on the panel raised questions about how serious a threat al Shabaab was to the US homeland. "While I acknowledge that the intelligence community sees a need to monitor al Shabaab's activities, I also know that vigilance must be in direct proportion to the probability and likelihood of the threat," said Bennie Thompson, the top Democrat on the panel. "Al Shabaab does not appear to present any danger to this homeland." On Tuesday, President Barack Obama's nominee to head the National Counterterrorism Center, Matt Olsen, warned that Al-Qaeda operatives in Somalia represented a significant threat. "I would say that beyond Al-Qaeda senior leadership in Pakistan, its presence in Yemen, that probably the next most significant terrorist threat may emanate from the Al-Qaeda presence in Somalia in terms of the willingness and apparent ability, or at least the intent, to strike outside of that particular country," Olsen told a Senate committee.