On Monday, three firefighters who served on 9/11 died within hours of each other, according to the BBC. Lieutenant Howard Bischoff, 58, and firefighters Robert Leaver, 56, and Daniel Heglund, 58, all died of cancer, though it's unclear if their illnesses were linked to their service after the attacks.

Fire Commissioner Daniel Nigro told the BBC that the firefighters' deaths are a "painful reminder that 13 years later we continue to pay a terrible price for the department's heroic efforts." All three were first responders who were exposed to toxic dust in the wake of the attacks.

Leaver and Bischoff were "lifelong best friends" who grew up together in Park Slope. Leaver's widow, Rosaria, told the New York Daily News: "I felt at least Robert and Howie went off to heaven together."

Heglund's brother, FDNY Capt. Paul Heglund, was also a first responder. "We would do 24 hours on and 24 hours off back then, but after working at the firehouse, you would go down there, work 10-12 hours and then go home to show your face," he told the Daily News. "Then it was back to the firehouse."

According to the BBC, about 1,000 deaths have been linked to "illnesses caused by toxic dust issuing from wreckage at Ground Zero."

[Photo via AP]