Firefighters perished trying to save others in the Twin Towers
Ms Jowell, who has been given special responsibility for co-ordinating the support for the British relatives of the victims of the attacks, also delivered personal letters of thanks from Tony Blair.
The prime minister had intended to meet firefighters and police during his visit to New York last week, but had to cancel that part of his visit to meet President George Bush.
About 200 British people are feared to have died in the terrorist atrocities, but Ms Jowell said it was still too soon to say exactly how many.
She said: "British families owe an enormous debt of gratitude to these firefighters, who lost 10 of their members when the Twin Towers collapsed.
"The Prime Minister wanted to come and thank them personally last week and I have brought a personal letter to the firefighters and the police from him.
"They need to know how grateful we are for what they have done to save the lives of British people in this unprecedented tragedy."
Firefighters gratitude
Ms Jowell was introduced to firemen Tom Woods and Captain Mark Munnelly, who met Tony Blair's wife Cherie when they visited the disaster scene last week.
Capt Munnelly said both visits had meant a lot to him and his men.
He said: "We know we are not isolated, but the whole world recognises what has happened and supports us.
Mr Blair's letter was sent to fire battalion chief James Wendling with a message of praise for the courage of his staff.
He said: "Words cannot express the sorrow we feel at the loss of your firefighters, nor can we express in sufficient terms our admiration for your bravery."
He said they were an "inspiration to the world", adding: "We stand with you in your sorrow".
His letter to police chief Kevin Ward said the courage of the officers was "a beacon to the world".
Family suppport
Later, Ms Jowell visited the family centre set up by authorities in New York to deal with the relatives and friends of those missing and killed in the World Trade Centre attacks.
She said: "It is an extraordinarily impressive operation on a very large scale.
"The family centre itself is like a great hangar, an immense space that has been kitted out with facilities for families."
And the Minister said she was using her visit to review whether there were "gaps" in the help being offered to British families.
"Whether or not the support is enough is something each family will judge," she said.
"We have put in place a whole range of measures. The major organisation concerned with supporting families facing bereavement have come together to help.
"One of my reasons for being here is to make sure that if there are gaps then we take action to fill them."