Missing posters have been put up around New York
Rick Rescorla, 62, who was born in Hayle, Cornwall, was the vice-president for security at Morgan Stanley Dean Witter, based on the 100th floor of the south tower.
Survivors of the attack have described how he sang as he organised the evacuation of the company's 3,800 staff, in an attempt to raise spirits and ease panic.
Bob Sloss, managing director of Morgan Stanley Dean Witter, said he last saw Mr Rescorla on the 10th floor of the tower ushering people to safety.
"He was definitely there well after it was established that the building was in trouble," he said.
"He was seen by many people and was singing to them to ease the panic."
Mr Rescorla's wife, Susan, said he rang her as he tried to evacuate his staff and said: "If anything happens to me I want you to know you made my life."
She said he regarded himself as a Cornishman first and foremost and "knew every song in the world and loved to sing".
"He made me smile and laugh. He was just a wonderful, wonderful man," she added.
During the 1993 bombing of the World Trade Center, Mr Rescorla, who moved to the US when he was 18, was said to be the last person to leave his tower.
Colleagues said he dropped his trousers to get the attention of the panic-stricken crowd he was trying to control.
His mother Annie and many of his family still live in Hayle.
Helping others
Michael Egan 51, from Hull, is believed to have died as he helped colleagues escape from the south tower - the second structure to be hit by a hijacked plane.
The vice-president of the multinational insurer AON, he is thought to have made three journeys back into the smouldering building to guide his staff down from their offices on the 100th floor.
His sister Christine, 55, who worked as a nurse in Canada, was visiting her brother's office on Tuesday morning as part of her holiday and is also missing.
Mr Egan's cousin, Ros Carr, said no one had heard from the businessman since he rang his wife to tell her he loved her, before heading for the offices for a third time.
Vincent Wells, who would have celebrated his 23rd birthday on Monday, is the youngest known Briton on the official US list of missing people.
He was working for London-based broker Cantor Fitzgerald on the 105th floor of the northern tower when the first plane struck.
His parents, Charles and Julia, were at home in Ilford, Essex, on Saturday when they were told that their son was officially missing. They have flown to New York.