(CNN) -- United Airlines blamed a technical glitch Wednesday for the brief return of flight numbers 93 and 175, designations retired after the September 11, 2001, attacks on New York and Washington.
Airline spokesman Rahsaan Johnson said the glitch occurred Monday, when a computer assigned those numbers to existing Continental Airlines flights. But he told CNN the numbers will remain off the books, and that United apologizes for the error.
"It should not have happened, and we should have caught the mistake sooner than we did," Johnson said. United and Continental are the process of merging, and flights will carry both United and Continental numbers, he said.
The two United flights were among four hijacked jetliners used as missiles in the worst terrorist attacks on U.S. soil. Al Qaeda operatives plunged Flight 175 into the south tower of the World Trade Center, while Flight 93 crashed into a Pennsylvania field after its passengers attempted to resist their hijackers. The union that represents United's flight attendants criticized the revival of those numbers, calling it "a terrible misstep."
"If they're backing off, then good. It should never have happened in the first place," said Sara Nelson, a spokeswoman for the Association of Flight Attendants. "We applaud any steps taken to ensure they're permanently retired, and that this doesn't happen again."
Johnson said United had discovered and corrected the problem before complaints emerged.
Nearly 3,000 people died in the 9/11 attacks, including the flight crews and passengers aboard all four jets. U.S. forces killed al Qaeda leader Osama bin Laden, the man behind the massacre, in Pakistan in early May.
CNN's Kara Devlin and Matt Smith contributed to this report.