NEW YORK (CBSNewYork) — A retired NYPD sergeant has penned a book about the story behind the 9/11 investigation.

“From Landfill to Hallowed Ground” looks at the search for evidence after it was removed from Ground Zero.

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For five months after the Sept. 11 terror attacks, retired Sgt. Frank Marra worked at the Staten Island landfill, sorting through 1.6 billion tons of debris.

“Dump trucks would come up with debris, unload them and here we were with rake and shovel,” Marra told WCBS 880’s Peter Haskell.

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The search was for any kind of evidence, including human remains.

“People who lost someone and they’ve never recovered anything, they’re at levels of heartbreak that are just totally unimaginable,” Marra said.

Now retired, the 48-year-old feels this is an important story to tell and found the writing therapeutic.

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“Other than the Tri-State area, we felt the rest of America might be forgetting about what happened on that day,” Marra said. “The book was a great avenue for me to release the thoughts that I did have.”