Three retired firefighters who worked at ground zero died on the same day from cancer, an illness that many fear might be connected to toxic World Trade Center dust released on Sept. 11, fire officials said Thursday.

Lt. Howard Bischoff, 58, and firefighters Robert Leaver, 56, and Daniel Heglund, 58, died within hours of one another Monday.

Their deaths are "a painful reminder that 13 years later we continue to pay a terrible price for the department's heroic efforts," Fire Commissioner Daniel Nigro said in a statement.

Thousands of people who aided in the rescue and recovery effort were diagnosed with respiratory ailments and other health problems in the years after the attacks. Cancer, though, remains the biggest fear for people exposed to the gritty soot at the site.

Hundreds of first responders have gotten cancer in the 13 years since the attacks, but doctors and researchers are still uncertain whether there is any link between those illnesses and 9/11. Cancer is the leading cause of death for Americans in their mid-40s to mid-60s, making it hard to tell which deaths, if any, might be related. Most medical studies have not found evidence of a substantial surge in cancer rates, though researchers have spotted some worrisome trends.

Congress has set aside $2.78 billion to compensate people with illnesses that might be related to the attacks. Administrators of the fund have included the most common types of cancer as qualifying illnesses.

"On that day when first responders arrived, the air was toxic and remained toxic for many months afterward," said Jake Lemonda, president of the Uniformed Fire Officers Association.

The Fire Department of New York lost 343 firefighters on 9/11. The department maintains a memorial to 89 other firefighters it believes died of illnesses. That tally doesn't yet include Bischoff, Leaver or Heglund.

Their deaths come as advocates urge Congress to reauthorize the James Zadroga 9/11 Health and Compensation Act, which provides medical treatment and compensation to those who got sick from exposure to toxic air after Sept. 11.

Fire officials knew the three were sick, said Lemonda, whose union represents fire lieutenants, captains, battalion chiefs, deputy chiefs, medical officers and supervising fire marshals in the FDNY. One had leukemia, one had esophageal cancer and the third had colon cancer.

Funerals for Leaver and Heglund were scheduled for Friday. The service for Leaver will be held at Francis of Assisi Church in West Nyack at 10 a.m. Heglund's funeral will be at the Centerport Volunteer Firehouse at 10:30 a.m.

A funeral for Bischoff will be held at 11 a.m. Saturday at St. Aloysius Church in Jackson, New Jersey.