WASHINGTON — U.S. President Joe Biden’s administration slashed the prisoner population at Guantanamo Bay detention center in Cuba by nearly half on Monday, sending 11 detainees to Oman. The U.S. military said only 15 detainees remained there after the transfer, following a major push toward closing the facility by Biden’s administration in its final days in office. The detention center was first opened on Jan. 11, 2002, by President George W. Bush to hold terrorism suspects and “illegal enemy combatants” during the U.S. “War on Terror” following the Sept. 11, 2001, attacks on New York and Washington. Guantanamo Bay housed