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By Natalie Dreier, Cox Media Group National Content Desk

Fifteen years after the horrific attacks and aftermath of the Sept. 11, 2001, attacks, a father and son have succumbed to cancer within months of each other.

Their deaths are being attributed to cancer that has been tied to ground zero, CNN reported.

Robert Alexander, 43, died this week. His father Raymond Alexander, 76, died nine months ago.

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Raymond Alexander was a New York City firefighter when the planes struck the World Trade Center. Robert was a member of the NYPD at the time, CNN reported.

Robert eventually became a firefighter for the FDNY.

In 2003, Raymond became ill and from 2003 to 2016, he was diagnosed with seven different types of cancer. Two years ago, their son Robert was diagnosed with inoperable brain cancer.

Raymond and Robert are the first father and son first responders to die of 9/11-related illnesses, the Uniformed Firefighters Association told CNN.

The union said that the cancers came from the air responders breathed during and after the attacks. The air had ash, debris and carcinogens in it.

“We had firefighters show symptoms very early after 9/11 in age groups that they shouldn’t have been showing symptoms for different diseases, specifically cancer,” Gerald Fitzgerald, UFA presidenttold CNN.

The Centers for Disease Control and Prevention has set up the World Trade Center Health Program that monitors and provides treatment for first responders and others who were involved in rescue and recovery at the ground zero site.

As of June, 7,139 people enrolled and certified with cancer have been covered by the program.