United Airlines has apologised for temporarily reviving the flight numbers of two hijacked planes used in the September 11 attacks.
The airline said the move was inadvertent and gave assurances that they would not be used again.
Numbers 93 and 175 were mistakenly assigned to two flights by Continental Airlines, which merged with United in October.
According to Reuters, United declined to explain how the numbers found their way back onto flights.
The news agency said flight attendants had urged the company to permanently retire them, while pilots said the move was insensitive.
Flight 175, a Boeing 767, took off from Boston and was one of two planes flown into New York’s World Trade Centre.
Flight 93, a Boeing 757, took off from New Jersey’s Newark International Airport and crashed into a Pennsylvania field.
It was believed the hijackers of that plane had intended to strike the White House.
Two other planes used in the attack were flown by American Airlines. Flight 11, a 767 from Boston, hit the World Trade Centre, and Flight 77 from Washington Dulles, crashed into the Pentagon.
Following the attacks, American Airlines retired those two flight numbers.
