Irish-American friends of Sinn Féin president Mr Gerry Adams are feared to be among the dead in New York.
They include Father Michael Judge, chaplain of the New York Fire Service, whom he had met several times, and a man who once organised a lunch on the top floor of the World Trade Centre to raise money for the party in Ireland.
Mr Adams said: "I'm told by friends in New York that there is little hope of finding him alive. Others we met that day are almost certainly part of the casualty list."
A three-minute silence is to be held in Belfast and Derry tomorrow as a mark of respect by the people of Northern Ireland. Special books of condolence have also been opened.
At a meeting of the Stormont Assembly today, which was called to pass a motion condemning the atrocity, the Democratic Unionist Party leader, the Rev Ian Paisley, walked out in protest when Mr Adams got to his feet.
Mr Paisley said he and his party were not prepared to listen the Sinn Féin president's "crocodile tears".
He said: "The great tragedy is that through concessions, terrorism has been made respectable and through concessions, terrorism has decided that it can take on the world."
During today's debate, Acting First Minister Sir Reg Empey revealed how a relative of his ministerial and party colleague Mr Michael McGimpsey, narrowly escaped death in the New York tragedy.
Mr McGimpsey's nephew Jason, a New York fireman, was on the 19th floor of the north tower of the World Trade Centre searching for survivors when the second jet, United Airlines Flight 175 flew straight into the south tower.
"Jason managed to get out in time before the huge skyscraper collapsed.
"Regrettably 250 of his colleagues weren't so fortunate," said Sir Reg.
He said the terrible events in the United States had particular resonance among people in Northern Ireland who knew only too well what it meant to endure terrorist violence.
Acting Deputy First Minister Mr Seamus Mallon said the world was shocked by the sheer horror of the scale of destruction visited on the United States.
He said: "This wasn't just an attack on America but an indiscriminate attack on a centre of world commerce used and staffed by people of all nationalities.
"This was an attack on us all."
PA