There is not a time in the life that I can remember that did not include Wesley Mittman. She was always there, a blazing point on the map of my social world, even if she was off living her life while I was living mine. We were Upper East Side kids, born two weeks apart. We met the summer we both turned four, in 1985, at a nursery-school camp at the 92nd Street Y. I can picture her face then clearly: small, bunchy, lit up with wild, happy eyes and an outsize smile, haloed with Slinky-like curls of dirty-blond hair. After that, we went to elementary school through high school together at Horace Mann, in Riverdale, where she grew into a five-foot dynamo. She studied hard, got sterling grades, and seemed to excel at anything put in her path. We ended up being college classmates, at Yale, and by the time we graduated we’d spent the entirety of our education together. Our journeys through life ran on parallel tracks, and I assumed that they always would.

Last Monday, a twenty-seven-year-old man who had driven from Las Vegas walked into the office tower at 345 Park Avenue with an assault rifle he’d bought from his supervisor at a casino and killed five people. They included a police officer named Didarul Islam; Aland Etienne, an unarmed security guard; Julia Hyman, a young employee at Rudin Management; and Wesley, who was an executive at the investment firm Blackstone. The fifth person he killed was himself. It was the deadliest shooting in New York City in twenty-five years. A note found in the gunman’s wallet indicated that he had suffered debilitating brain trauma from playing high-school football and had come to target the National Football League, which is headquartered in the same building. He wanted his brain studied.

I had seen news of the shooting that evening but didn’t know that Wesley—who went by her married name, LePatner—was one of the victims until the next morning, when I woke up to a text from an old friend. It seemed astonishing that Wesley, of all the people in New York, would be in such the wrong place at such the wrong time. She was all about doing everything exactly right. In high school, she was our very own Tracy Flick: brilliant and confident and overachieving—but not in a way that anyone resented, because her smile was so big, and her voice was so raspy and warm. And she seemed to like everyone, even as she breezed past them in the private-school rat race. Wesley would rack up accomplishments with a sparkling air of perfection, then march cheerily forward. There’s no story to tell about her that makes sense with this ending.

There are people we know who are less than friends but more than acquaintances, people who exist as fixed points in the worlds that made us. I hadn’t seen Wesley much after our school years, but I can picture her at every age, and I can see her trajectory of forty-three years in a flash. My favorite Wesley story is from tenth grade, when our entire class was gathered for our first meeting with the school’s college counsellor, Mr. Singer. Mr. Singer had a dry, Walter Matthau-like affect, and he started off by saying, “The first thing you need to know is that none of you have to worry about college applications yet . . . Mittman.” Everyone laughed. Of course she had knocked on his office door already, Lord knows how many times. The next year, my friend Alice, a girl of quiet intelligence, was in heated contention with Wesley to be editor-in-chief of the student newspaper. Of course Wesley got it. She had the loud intelligence of a born leader, as charismatic as she was studious. (Alice remembers her now as “competitive but kind.”) In the yearbook, she quoted Teddy Roosevelt, Jane Austen, and Ferris Bueller. She was unstoppable.

I saw less of Wesley in college, in part because she met her future husband, Evan LePatner, on the first day of school, and they were rarely apart. She graduated, naturally, summa cum laude. (I’d decided, after the pressure cooker of high school, not to sweat over grades, and graduated nada cum zilch.) After school, she started working at Goldman Sachs, and she married Evan in 2006. I would see her at reunions; once, in our twenties, she told me that they were living in the West Village, which she described as “a great place to live when you’re young.” It struck me as something you would say only if you had an entire life plan sketched out. If our lives were playing out in parallel, I thought of Wesley’s as an unwavering straight line: whatever environment she entered, she would ace it. I could trace my choices by their deviations, however slight or large, from Wesley’s example. By the time we were in our forties, she had a C-suite job, two children, and seats on various boards (the U.J.A.-Federation of New York, the Metropolitan Museum). She was ensconced in the New York power center, while I had made my career as an observer, a writer—not a bohemian choice by a long shot, but I could measure the degrees of distance.

The last time I saw Wesley was at a college reunion two years ago. She said that she wanted to introduce me to a classmate she was certain I should write about, then yanked me by the wrist over to meet him. It was quintessential Wesley: assertive yet charming, driven by a belief in her power to make things happen. (No, I didn’t write about him. But I can’t deny Wesley’s last request to me, so let me tell you now about Brian Wallach, a lawyer who worked in the Obama White House and then became a patient advocate for A.L.S. after his own diagnosis, and also the subject of the documentary “For Love & Life: No Ordinary Campaign.” I’m sorry it took me this long.)

In the days after the shooting, I started hearing from shocked classmates. The schools we attended sent out letters. I’ve had classmates die, tragically and too young, but not in a mass shooting that put the whole city on edge. Wesley was part of a news event, and her face appeared in obituaries, which described her as a beloved mother and mentor and executive and philanthropist. (Fortune’s obit was written by A.I., which made things feel even more unreal.) A friend texted photos of Wesley and me at a first-grade sleepover; I didn’t remember the sleepover, but I remember Wesley’s buoyancy at age six. There was something primal about losing this person who shared so much of my history, and why? Because she’d left work at a particular time—not five minutes earlier, not five minutes later?

This past Thursday, I went to Central Synagogue, just a few blocks from where Wesley was killed, to attend her funeral. Camera crews were penned in across Lexington Avenue; the temple was packed to the gills. In the aisles, I saw faces I’d known for decades, easier to call up in their ten-year-old forms than in their current, middle-aged ones. We hugged, as if at a grim class reunion. There was very little to say, except that Wesley seemed like the last person this would happen to. Someone had seen her just a few days earlier, when she’d hosted an event for the Audubon Society, in support of her daughter’s passion for animals.

The eulogies lasted more than two hours, and they revealed aspects of Wesley’s life I hadn’t known. She was devotedly Jewish; at her father’s suggestion, she’d spent the summer between high school and college studying the Talmud at an institute that allowed women to do so. As a go-getting junior analyst at Goldman Sachs, she had e-mailed the highest-ranking woman at the firm to introduce herself, and didn’t get a reply—but as her own star rose she’d made a point of mentoring younger women. When she was recruited by Blackstone in 2014, she’d struggled with the decision, and she agreed to the job under the condition that she be home to put her kids to bed every night. Her husband recalled her, when they met as college freshmen, as a “crazy ball of atomic energy”; when he offered to help her set up her computer, he was surprised to learn that she meant at seven-thirty in the morning. A horrible jolt came midway through the remembrances, when a fourteen-year-old girl who looked unsettlingly like fourteen-year-old Wesley got up and spoke, in a high, trembling voice, about losing her mother.