United Airlines temporarily revived the flight numbers of two hijacked planes used in the Sept. 11 attacks but said Wednesday the move was inadvertent and they would not be used.
"We are taking immediate steps to remove them and apologize for the error," a United spokesman said.
Call numbers 93 and 175 were mistakenly assigned to two flights by Continental Airlines, which has merged with United.
United declined to explain how the numbers found their way back onto flights. Unionized flight attendants urged the company to permanently retire them, while pilots said the move was insensitive.
Flight 93, a Boeing 757, took off from New Jersey's Newark International Airport and crashed into a Pennsylvania field. Investigators believed the hijackers of that plane wanted to hit the U.S. Capitol or the White House.
AMR Corp, parent of American Airlines, long ago retired those two flight numbers.
Other airlines, however, continue to use all four of the numbers for their flights.