NEW YORK • Nearly four years ago, novelist Wendy Mills was at the airport with her family when her son Zack asked a simple but wrenching question.
He wanted to know why all the passengers going through security were taking off their shoes. Mills gave Zack, then nine, a vague reply such as "It's to keep us safe".
Unsatisfied, he persisted, and eventually, the conversation wove around to the Sept 11 attacks, which happened before he was born.
Mills, who lives in Pine Island, Florida, and writes young-adult fiction, realised during that conversation that most of her teenage readers had no direct memory of the attacks.
Even though you see videos and hear in history class about what happened, I felt I never understood how people felt... but now I do.
NEW JERSEY HIGH SCHOOL STUDENT JUSTIN GENSCH,14, on how reading The Memory Of Things gave him a sense of what it was like on the morning the towers fell
Soon after, she started working on All We Have Left, a novel about two teenagers - a Muslim girl named Alia and a troubled boy called Travis - who are trapped in an elevator in the World Trade Center on the morning of Sept 11.
The novel jumps back and forth between 2001 and 2016, when Travis' younger sister, Jesse, tries to find out what happened to her brother, who died in the tower, and fights with her grief-stricken father, who has become virulently anti-Muslim.
"I wanted to write a story that made our shared history accessible to them," Mills said.
"Here's a whole generation of kids who weren't alive and don't know what it was like that day and they're not going to know the world before Sept 11. It wasn't a perfect world, but it felt like a safer world."
All We Have Left, which was published last month, is one of around a half-dozen Sept 11-themed novels for young readers coming out this year.
The cluster of books on the subject, arriving in time for the 15th anniversary of the events, represents the first major wave of children's literature that explicitly deals with the terrorist attacks and is aimed at readers with no memory of the events.
The stories range from tame middle-grade novels for eight- to 12-year-olds, which centre on the tragedy but keep the horror at a distance, to young-adult novels with unflinching descriptions of the mayhem and bloodshed that unfolded as the towers burnt and fell.
While Sept 11 has become a familiar plotline in film, television and adult fiction, children's book authors have been reluctant to take it on. Many worried that the material was too traumatic for young readers and feared that parents and teachers would be skittish about engaging with the subject.
Others thought using the attacks as a plot device might seem insensitive and exploitative.
"Ten years even felt too soon," said Ms Alvina Ling, vice-president and editor-in-chief of Little, Brown Children's.
"It's so painful for a lot of us."
Before this year, the handful of children's book authors who tackled the issue did so with trepidation.
"Writing a bad book is okay, but writing a bad Sept 11 book, that was terrifying," said David Levithan, whose young-adult novel about the attacks, Love Is The Higher Law, came out in 2009.
Now, another fear has taken hold. For many children who were not alive on Sept 11 or were too young to remember the attacks, the events feel distant and unreal, like a Hollywood disaster movie.
Some know only the broadest outline of the event, while others have been shielded from it altogether.
It is unclear whether there is a big appetite for these books among young readers, and some publishers remain wary.
Gae Polisner struggled to find a publisher for her new young adult novel, The Memory Of Things, which opens right after the towers fell and follows a teenager named Kyle as he flees downtown Manhattan and rescues a girl cowering at the side of the Brooklyn Bridge.
When Polisner and her agent sent the novel around to publishing houses two years ago, more than 10 editors rejected it.
Finally, an editor at St Martin's Griffin bought it. The novel, which was published on Tuesday, has drawn glowing early reviews and has been embraced by some teachers and school librarians.
Justin Gensch, a 14-year-old at David Brearley High School in Kenilworth, New Jersey, said that reading The Memory Of Things gave him a sense of what it was like on the morning the towers fell.
"Even though you see videos and hear in history class about what happened, I felt I never understood how people felt," he said. "It helped me understand what the fear was like because I never really got it, but now I do."