United Airlines is apologizing after briefly restarting use of flight numbers of two of the doomed planes hijacked on Sept. 11, 2001. Flight numbers 93 and 175 have been removed from the airline's computer system after a United flight attendants' union objected to the reuse. The flight numbers probably went back into use just in the past few days, and a spokesman blames a technical error. United Flight 93 crashed in Shanksville, Pa., and Flight 175 hit the south tower of the World Trade Center. (More United Airlines stories.)