Kenya's president said Tuesday that the Nairobi mall siege is over with five militants killed and 11 in custody in connection with the deadly attack, NBC News reported. Sixty-one civilians and six security officers were killed after the 4-day standoff, President Uhuru Kenyatta said. Some 62 people have been operated on in the wake of the siege and remain in the hospital, he also said. Three floors of the Westgate mall had collapsed and bodies remain in the rubble, he added. Kenyatta said that Wednesday marked the beginning of three days of national mourning. A senior U.S. law enforcement official, meanwhile, told NBC News that authorities have been unable to verify that Americans were among the attackers. Kenya's foreign minister has said "two or three" young Americans between 18 and 19 years old were believed to have been involved, as well as a British woman. Kenyatta did not confirm American or British involvement.