Dozens of New York firefighters rushed to the World Trade Center skyscraper early on Wednesday after a member of the public mistook welding for a fire.
Initial reports from the New York Fire Department suggested a minor blaze on the 88th floor of One World Trade Center, the centerpiece in the project to replace the Twin Towers destroyed in the 11 September, 2001 terrorist attacks.
A spokesman at first told AFP that there was a fire at the 104-storey building but that it had been brought under control in less than an hour.
Subsequently the department downgraded the incident, saying that the 84 firefighters had been dispatched to the still unfinished tower because a member of the public had reported seeing a fire.
In fact, there was no blaze.
The incident took place in an area where construction workers have been welding steel and the incident appeared to have been no more than "a flash," or other welding-related phenomenon, the spokesman said.
However this was not considered a false alarm. "There was an incident at the scene. Calling us on 911 was necessary," the department spokesman said.
Nearly 3000 people died during the 11 September 2001 attacks on the Twin Towers and security at the new site is always high.
The last floor of the new tower, briefly dubbed Freedom Tower, has been built and last week a steel beam signed by President Barack Obama was lifted into place.