In the years following the 9/11 terrorist attacks, Donald Trump worried that he might not be able to rent space in his skyscrapers.

"When you see pictures of people on fire jumping off the building when the World Trade Center was hit, you think: 'This may be hard to rent,'" Trump said in a May 11, 2005, interview with journalist Timothy O'Brien that was included in the book "TrumpNation: The Art of Being Donald Trump."

At the time of the interview, Trump was trying to build the tallest skyscraper in Chicago but facing resistance from people who "don't get it." He was also frustrated that Midwesterners wouldn't let him exaggerate the floor count as he had been able to do in New York, O'Brien wrote.

Trump frequently invokes 9/11 on the campaign trail, especially in defense of his proposal to temporarily ban the entry of Muslims and others from countries "compromised" by terrorism. During the Republican primary, Sen. Ted Cruz, Texas, attacked Trump's "New York values" during a debate, and Trump responded with an emotional remembrance of the terrorist attacks on his city.