HACKENSACK, N.J. -- The American flag that was raised by three firefighters over the wreckage of the World Trade Center, one of the most enduring images of Sept. 11, has disappeared.

After it was removed from the site during cleanup, the flag was believed to have been flown on U.S. ships serving in the war in Afghanistan, then returned to New York City officials in March.

But the flag that city officials preserved measures 5 feet by 8 feet. The flag the firefighters raised on Sept. 11 measured 4 feet by 6 feet, according to its original owners.

"It's just a really awkward and difficult situation," said Lark-Marie Anton, a spokeswoman for New York Mayor Michael Bloomberg. "What it represents is really what's important."

Bloomberg has asked city fire officials to investigate what happened to the flag.

The New York Times reported that city officials had traced the 5-by-8 flag as far back as a Sept. 23 appearance in Yankee Stadium.

They believe the original flag may have been accidentally switched or misplaced sometime between Sept. 11 and that event.

Officials say they are unaware of anyone claiming to possess the original.

The original flag came from a yacht, the Star of America, that was in a Hudson River marina near the World Trade Center that day. Firefighter Dan McWilliams took it from the yacht and walked back to Ground Zero, where he and two colleagues, George Johnson and Bill Eisengrein, raised it on a slanted pole.

The scene was captured by Thomas Franklin, a photographer with The Record of Bergen County, and distributed worldwide by The Associated Press.

The discrepancy about the flag size was discovered last month when the yacht owners, Shirley Dreifus and her husband, Spiros Kopelakis, borrowed the flag for an event on board the Star of America.

The couple had been preparing to formally donate the flag to the city when they said they noticed the flag was too big to be theirs.

"It's a mystery," Glen Oxton, an attorney representing the owners said Thursday in The Record. "Who knows what happened to it after the firefighters put it up and the photograph was taken? There was so much activity down there."