Image copyright AP Image caption From left: Lt Howard Bischoff and firefighters Daniel Heglund and Robert Leaver

Three firefighters who were on duty at Ground Zero during the 9/11 attacks have died on the same day from cancer, fire officials have said.

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

Thousands of people who helped the 9/11 rescue efforts have been diagnosed with illness, including cancer.

But doctors say it is unclear whether sickness can be linked to the attacks.

Fire Commissioner Daniel Nigro described the three men's deaths as a "painful reminder that 13 years later we continue to pay a terrible price for the department's heroic efforts".

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

Some 1,000 deaths have been linked to illnesses caused by toxic dust issuing from wreckage at Ground Zero.

Image copyright AP Image caption One month after the towers fell, the site was still thick with smoke

Image copyright AFP Image caption The first responders worked without masks for hours

Many of those who were on the site have suffered from respiratory illnesses, and hundreds of first responders have also contracted cancer.

But despite these numbers, no clear link between the attacks and cancer has been made.

Medical studies have not reported evidence of a substantial surge in cancer rates to those connected to 9/11, though government reports have suggested that workers were exposed to a number of chemicals that could cause cancer.

The Fire Department of New York has said that in addition to the 343 firefighters who died on the day of the attacks, a further 89 have died from illness.

Image copyright AP

The US Congress has set aside $2.78bn (£1.7bn) to compensate those with illnesses that might be related to the attacks.

But legislation which provides medical treatment and compensation to 9/11 victims is due to expire in two years, and Congress has yet to vote to extend it.

"I'm asking [leaders in Washington] to be as brave as the people who responded on that day," said Mr Lemonda.

"This is not just a firefighter issue. This is an American issue."