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 been diagnosed with cancer in the 13 years since the attacks, but doctors and researchers are still uncertain whether there is any link between those illnesses and the Sept. 11 attack and aftermath. 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 among 9/11 responders, 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.

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 related 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.