On the morning of September 11, 2001, eight-year-old Amy Gaines’ father, Scott, dropped her off at the school bus stop. It was supposed to be the first day of his last vacation before his retirement after 20 years as a New York City police officer.

But then, news broke that a plane — and then, a second one — had flown into the World Trade Center. So Scott Gaines headed to Ground Zero, where he would continue to work for the next two months.

Like many 9/11 first responders, he would later be diagnosed with cancer. In fact, the projected death toll from illnesses potentially linked to 9/11 is larger than the number of people who died that day.

In a new episode of The FRONTLINE Dispatch called “The Weight of Dust,” Amy Gaines — now a series coordinating producer at FRONTLINE — embarks on a deeply personal quest to understand the long arm of 9/11 through the story of what happened to her father and thousands of others diagnosed with illnesses believed to be caused by exposure to toxic chemicals at Ground Zero.

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It was October 2013. ISIS had splintered off from al-Qaeda earlier that year.

The militant group grows stronger and attracts recruits from all over the world, many from Western Europe.

Ayan and her younger sister Leila are Somali-Norwegian teenagers living in an affluent neighborhood outside of Oslo. They leave their adopted homeland to travel to Syria and marry ISIS fighters.

Author Asne Seierstad shares the family's story in her latest book, "Two Sisters: A Father, His Daughters and Their Journey into the Syrian Jihad."

At first, says Seierstad, Ayan and Leila's mother is worried that the girls are becoming "too Norwegian" and sends them to classes with a charismatic Quran teacher. And that's when the parents notice a change.

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