The memo went out Friday afternoon in a skyscraper that houses the offices of several magazine publishers, including Reader's Digest, Conde Nast and The Economist.
The message warned workers that a gunman was in the building and that shots had been fired on the 10th floor. They were urged to get behind a locked door or lay down on the floor and keep quiet.
Building manager James Shanahan said that it was all a mistake and the memo was meant for a drill.
A few minutes later, another e-mail went out with an apology.
In the meantime, some workers locked themselves in a closet and others dialed 911.