Did we really need a dramatic recreation of what happened aboard United Airlines Flight 93 on the morning of Sept. 11, 2001? Not really. Those who clamor about the movie’s necessity, that it will ensure we “never forget” the tragic events of that day, must have short memories, indeed. It’s been 60 years since the attack on Pearl Harbor, and yet we still remember that every year, often without the aid of a movie.

So, if we don’t need “United 93” to remind us of the events of 9/11 (and I don’t think it’s hyperbolic to suggest that just about everyone in America remembers, and continues to think about, that day), then what do we need it for? We’ve already got plenty of memorials and observances, and the odyssey of the passengers of United Airlines flight 93 is writ large in the official report of the 9/11 Commission. While the purpose of the passengers, and, for that matter, the terrorists, aboard the plane was single-minded and clear, the purpose of “United 93” remains murky.

Those who are worried that the film, part memorial and part documentary, is exploitive or crassly commercial needn’t fear. Director Paul Greengrass seems to have gone out of his way to make “United 93” as understated and somber as possible. Even the now-legendary line “Let’s roll” is a barely audible aside, uttered more with impatience than dramatic flair.

However, “United 93” has drama to spare. Even though the audience knows how events will unfold, the film carries with it a heavy dose of suspense. The first hour juxtaposes the pre-flight routines of the passengers, crew members and hijackers with the abnormal chaos happening at various air traffic control centers across the country. On the morning of 9/11, while United 93 sat on the runway awaiting take off, the two planes that hit the World Trade Center had already been hijacked and the FAA, NORAD and other air traffic centers were in full crisis mode.

Once the hijackers take over United 93, the film’s focus shifts entirely to the plane. Here, the film is absolutely harrowing and unnerving. By the time a group of passengers make their bid to regain control of the plane, “United 93” switches from a slow-burn style narrative to a rush of violence, adrenaline and panic that culminates with a chilling shot from inside the cockpit of the rapidly approaching ground. It feels a little like getting punched in the stomach.

That the film gains such an emotional hold over the viewer is unexpected. Because Greengrass launches us directly into the events of the day, there’s not even time to learn anyone’s name. Some snippets of dialogue tell us who has kids, who’s on vacation and whatnot. But the only sense we get of the passengers as real people comes from the eerily natural performance all the actors give. The sole character whose name is mentioned is Ben Sliney, who, on the morning of 9/11, took over as the national operations manager for the FAA. In “United 93,” Sliney stars as himself. In fact, a handful of air traffic controllers who were working that day portray themselves in the film.

That blurring of fiction and reality is what makes “United 93” emotionally engaging. It feels real and looks real and, worst of all, the events portrayed are real. Because it’s a film, we hope for some kind of positive outcome. Maybe the one hijacker, who seems so hesitant to go through with the plan, will change his mind. Maybe the passengers will get control of the plane and land it safely. But because “United 93” is also a piece of recent history, the memories still burn bright and clear. And so, the sense of empathy that comes with good storytelling blends with our own memories and the effect is devastating.

Greengrass’ stark direction and a fairly straightforward account of events, along with the use of actual participants in the day’s events in the film, gives the film the feel of a first-class documentary. But there’s also a strong narrative thread, albeit a bleak one, the resolution of which continues to play itself out in the news every day. It’s not so much rousing and inspirational as it is heartbreaking and depressing.

Did we need “United 93”? Not really. As nonfiction, “United 93” offers a plausible account of what happened on the morning of 9/11. But as a piece of fiction, “United 93” only sharpens already painful memories without offering much comfort.