City officials on Thursday launched a follow-up survey for thousands of people in a registry tracking post-Sept. 11 health problems, looking for proof of persistent respiratory and psychological illnesses in those who worked or lived near ground zero.

The World Trade Center Health Registry gathered initial information from 71,437 people who worked at ground zero or were in the area at the time of the attacks, making it the nation's largest such registry.

The program has come under criticism for failing to reach conclusions about post-Sept. 11 health effects sooner and for not providing information about treatment to survivors.

"We wish we had all of the answers. We wish we knew what the long-term health effects of 9/11 are," Health Commissioner Thomas Frieden said Thursday. "But we don't."

The new questionnaires ask for more details about the registrants' exposure and any update on their symptoms.

Officials have warned that it may take 20 years before doctors know what Sept. 11 did — and did not do — to the emergency personnel, civilians and others engulfed in the airborne remains of the two 110-story buildings.

A class-action lawsuit representing thousands of ailing workers and civilians blames Sept. 11 for their health problems, and two programs in the city are treating tens of thousands of rescue workers who say they developed sinusitis, cancers and other ailments after the attack.

In addition, a New Jersey medical examiner this year declared that the death of a retired city police detective who spent hours at the trade center site was "directly related to the 9/11 incident."

Frieden said that while registrants have reported respiratory problems, post-traumatic stress disorder and anxiety, "what we don't know are how long the symptoms are going to persist."

He said it would take several months to gather the data and that a follow-up report could be completed by the end of the year.