Less than two weeks after the 13th anniversary of the September 11 terrorist attacks, three New York firefighters who worked at Ground Zero died of cancer, all on the same day, the Fire Department of New York said.

Lt. Howard Bischoff, 58, and firefighters Robert Leaver, 56, and Daniel Heglund, 58, died within hours of one another Monday. Bischoff had colon cancer, Leaver had leukemia and Heglund had esophageal cancer. Many believe the diseases were related to their time serving as first responders at the World Trade Center in the immediate aftermath of 9/11. FDNY officials say 850 firefighters and ambulance workers at the scene have been diagnosed with cancer since then, related to that work at Ground Zero.

Heroes to the end: Three retired @FDNY members die of Sept. 11th-related cancers on same day. http://t.co/vVkaKfL9enpic.twitter.com/FmwdZKNDHN — New York Daily News (@NYDailyNews) September 25, 2014

"Losing three firefighters on the same day to WTC-related illnesses is a painful reminder that, 13 years later, we continue to pay a terrible price for the Department's heroic efforts on September 11th," Fire Commissioner Daniel Nigro said in a statement.

The FDNY lost 343 firefighters on 9/11 and 92 more since then ‒ including the three men ‒ due to illnesses directly related to contaminated air at Ground Zero, CBS News reported.

"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, according to CBS News.

First responders began showing health problems soon after the attacks, with 99 percent of exposed firefighters reporting at least one new respiratory illness, according to NYC Resources 9/11 Health Statistics.

“When he was diagnosed, Robert said he was not a victim but a first responder,” Leaver’s widow, Rosaria, told the New York Post. Her husband was diagnosed in 2003, and retired after serving as a firefighter with Engine 202 in Brooklyn for 20 years. “He said the real victims were the civilians who weren’t first responders. That’s the kind of guy Robert was.”

Leaver and 19-year veteran Bischoff grew up together in Brooklyn, and were close friends. “I felt at least Robert and Howie went off to heaven together,” Rosaria told the New York Daily News.

"To learn that Bobby Leaver who grew up five doors down and was best friends with my brother and our family, passed on the same day, that was really tragic," Bischoff ‘s brother John told WABC.

Retired @FDNY FF Robert Leaver was a member of Engine 202 in Red Hook. He died of WTC-related cancer on Monday. @NY1pic.twitter.com/saObCzM4t4 — Mahsa Saeidi (@MahsaWho) September 25, 2014

"We had the diagnosis of incurable cancer three years ago," John added. The pronouncement of stage 4 colon cancer caused lieutenant from Ladder 149 to really start living. "The past three years, particularly, there was an extra emphasis on going to Europe, which was not part of my brother's plan. Just so the kids could see more of the world while he was seeing it with them.”

Heglund, who would have been 59 on Tuesday, served as a firefighter for 21 years, most recently with Rescue 4 in Queens. His esophageal cancer had metastasized into his bones.

“He suffered. He’s been fighting. He put up a good fight,” Heglund’s brother, FDNY Capt. Paul Heglund, told the Daily News. He said his brother told him before he died that he had no regrets about working at Ground Zero.

“About 99.9% of us wouldn’t change anything that happened. Even after 9/11 and what went on afterwards, we were firemen,” Capt. Heglund added. “That's what we do.”

"I think we're processing the irony of three men dying of cancer so long after 9/11, and I hope they got to spend the last couple of years the same as my brother did. Surrounded by family and friends," John Bischoff said.

Retired @FDNY Lt. Howard Bischoff, of Ladder 149 in Dyker Heights, also died of WTC-related cancer on Monday @NY1pic.twitter.com/MdSnGRzTLx — Mahsa Saeidi (@MahsaWho) September 25, 2014

The three deaths come as advocates for the first responders call on Congress to reauthorize the James Zadroga 9/11 Health and Compensation Act. The law was passed in 2010, and helped to ensure proper monitoring and treatment for first responders and survivors who face potential life-threatening health effects as a result of the attacks.

"When we responded we were told the air was safe, don't worry," Lemonda told WABC.

More than 30,000 people are currently receiving treatment through the WTC Health Program, according to the office of Sen. Kirsten Gillibrand (D-NY). Most of the treatment is for respiratory illnesses, including asthma.

If the James Zadroga Act is reauthorized, the health care program would be extended through 2041. The legislation is named for an NYPD detective who died after working at Ground Zero.