Starring Ben Sliney, J.J. Johnson, and Khalid Abdalla. Rated 14A.

Witnessing the unthinkable horrors that unfolded on CNN on September 11, 2001, it was impossible not to wonder what it would have been like to be trapped aboard one of the hijacked airliners. In his controversial new film, writer-director Paul Greengrass uses authentic detail to put us on one of the doomed flights””the only one where the passengers had the added terror of knowing about the World Trade Center attacks. Watching it is an agonizing experience. But thanks to Greengrass's intelligence and careful avoidance of melodrama or graphic violence, it is also an insightful one. Yes, United 93 actually manages to rise above being the morbid curiosity or flag-waving propaganda it first sounds like.

The director heightens the you-are-there feeling by using handheld cameras and shooting the 90-minute flight in real time. He also casts a mixture of unknown actors and people who play themselves, from air-traffic controllers to flight attendants (including the Federal Aviation Administration's charismatic national operations manager, Ben Sliney).

Greengrass bases his script on research into the 9/11 Commission report and interviews with family members (who threw their support behind the film) to patch together the chain of events that allowed a delayed passenger jet to take off at 8:42 a.m. and remain in the air long after the crash at the north twin tower at 8:46 a.m.

Greengrass knows such events need no exaggeration, and his skill is in showing the very averageness of the people involved. Early on, the dread is overwhelming watching flight attendants gossip and passengers yawn in the waiting room. All are utterly oblivious that Flight 93 out of Newark will dive into a Pennsylvania field en route to Capitol Hill and that the world is about to change.

This quest for authenticity ends up challenging the more official portrayals of heroes and villains. The passengers, initially paralyzed by fear, are moved by equal parts panic and survival instinct, not some grand plan to die for their country. The terrorists are not cartoon bad guys but often seem as scared as their captives. Greengrass even avoids jingoism in that famous call of “Let's roll!” ; seen here, it's a brief outburst lost in the frantic blur of an attack.

There is just as much intensity in the air- traffic control room, where idle chitchat about how there hasn't been a hijacking in 20 years turns to full-blown chaos. Meanwhile, commanders at the Northeast Air Defense Sector can't round up more than a few unarmed jets. A consistent breakdown in communication, paired with the inability to grasp the full horror of the situation, sealed United 93's passengers' fates.

Questions will continue to dog the film. Why make it? And why would potential viewers want to subject themselves to it? Greengrass, who made an equally incisive portrait of Ireland's Bloody Sunday, raises important questions about the system failures that allowed the flights to go down. Ultimately, though, his film works on a nonpolitical level: it's a strange, often upsetting, mix of memorial and catharsis, one that forces viewers to consider what they would have been willing to do on that flight. Whether or not that's a trip you take is your choice.