As many as 100 detainees at the prison at Guantnamo Bay, Cuba, could end up held without trial on American soil, Defense Secretary Robert M. Gates suggested Thursday, a situation that he acknowledged would create widespread if not unanimous opposition in Congress. The estimate was the most specific yet from the Obama administration about how many of the 241 prisoners at Guantnamo could not be safely released, sent to other countries or appropriately tried in American courts. Mr. Gates said discussions had started this week with the Justice Department about determining how many of the Guantnamo detainees could not be sent to other countries or tried in courts. He did not say which detainees might be in that group, but independent experts have said it probably would include terrorism suspects whom the military has not yet brought charges against, among them detainees from Yemen and the Qaeda figure Abu Zubaydah, who was subjected to brutal interrogation in secret prisons run by the Central Intelligence Agency. He did not say ... under what law they would be held. The Obama administration is debating how to establish a legal basis for incarcerating detainees deemed too dangerous to be released but not appropriate to be tried because of potential problems posed by their harsh interrogations, the evidence against them or other issues. Mr. Gates said he had asked for $50 million in supplemental financing in case a facility needed to be built quickly for the detainees.