There has been considerable speculation about the identity of the militants and how they managed to pull off a sophisticated assault that killed at least 62 people and kept security forces at bay. Kenyan Foreign Minister Amina Mohamed said Monday that “two or three Americans” and “one Brit” were among the perpetrators of the attack.

She said in an interview with “PBS Newshour” that the Americans were 18 to 19 years old, of Somali or Arab origin and lived “in Minnesota and one other place” in the United States. The British jihadist was a woman who has “done this many times before,” Mohamed said.

U.S. officials said Monday that they were pressing to determine whether any of the assailants were American.

“But at this point we have no definitive evidence of the nationalities or identities of the perpetrators,” said State Department spokeswoman Jen Psaki.

Al-Shabab is a Somali militia. But Gen. Julius Karangi, chief of the Kenya Defense Forces, told reporters that the militants inside the mall were “clearly a multinational collection from all over the world,” though he did not offer details. “We are fighting global terrorism here,” Karangi said.