“That’s exactly the kind of person he is,” Yu said. “When we hear explosions of gunfire, a lot of people immediately jump away. His instinct is quite the opposite — he jumps straight at it.”

Spindler survived the deadliest terrorist attack on American soil, but he was killed Tuesday when terrorists stormed an office and hotel complex in an upscale neighborhood of Nairobi. He was one of at least 21 people who died in the attack by al-Shabab militants.

Yu said Spindler, the oldest of three brothers from Houston, was a “ball of energy” who served in a remote area of Peru as a Peace Corps volunteer and traveled the world in search of adventure and fulfillment. His life changed course after the Sept. 11, 2001, attacks. “Something struck a nerve and changed how he felt and thought about things,” Yu said. “He just felt like he could be doing so much more.”

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Spindler left investment banking, pursued a law degree at New York University and later moved abroad to focus on social entrepreneurship.

In a statement, the NYU School of Law said Spindler is “warmly remembered by those who knew him."

“His tragic death is a loss not only to his loved ones, but to the community of individuals dedicated to improving the lives of others through social enterprise,” the statement said.

The State Department confirmed that an American was killed in Tuesday’s attack but has not yet released a name.

On Facebook, Spindler’s brother Jonathan wrote, “There are no words to describe how our family is feeling but I can say . . . Jason Spindler you are and will always be an amazing son, brother, and uncle.”

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Jason was “a survivor of 9-11 and a fighter. I am sure he gave them hell!" Jonathan wrote.

Yu reiterated those sentiments. “I have no doubt that when he heard the explosions outside the hotel, he was one of those trying to jump in and help,” he said. “It would be unlike him if we heard that he sat on the benches and hid in the bathroom.”

Spindler’s family is flying to Kenya to retrieve his body and will hold a religious service for him Monday — what would have been his 41st birthday, Yu said.

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Names and information about other victims also started to trickle out Tuesday evening and Wednesday.

Abdalla Dahir and Feisal Ahmed, who were close friends and colleagues, were among the victims. Adam Smith International, the economic-advising company where they worked, said the two were “killed on the terrace of a restaurant in the complex” where the company’s Kenya office is located.

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They worked on a project called Somalia Stability Fund, which the statement described as an effort “to bring peace and prosperity to Somalia through more than 100 local community initiatives.”

The same company lost an adviser, James Thomas, in the 2013 terrorist attack on the Westgate mall in Nairobi, the statement said. Al-Shabab also claimed responsibility for that attack, which killed 67 people.

British charity Gatsby Africa confirmed that British citizen Luke Potter, its Africa programs director, was killed in Tuesday’s attack.

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"Luke was respected by all he worked with, bringing huge drive, determination, a relentless work ethic, and a thirst for new ideas to every project,” the organization said in a statement. He had spent the past decade “helping some of the poorest and most vulnerable people in the world.”

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Kenyan radio station Capital FM reported that James Oduor, nicknamed “Odu Cobra,” was one of those killed Tuesday. Oduor tweeted during the attack, saying he was among those “trapped in our buildings.”

“Gunshots and non-stop explosions,” he posted. Three minutes later, he wrote “Waaaah. What’s happening at 14 Riverside fam? Any news from out there?”

Then his account went silent.