According to Variety, Paul Greengrass has started shooting Flight 93 at London's Pinewood Studios for Working Title Films and Universal Pictures.

The semi-improvised movie tells the story of United Airlines 93, the last of the four aircraft hijacked on Sept. 11, 2001. Passenger action caused the plane to crash before it reached its target.

The pic will unspool in real time, tracking what was happening inside the aircraft as well as on the ground.

"9/11 was unique -- the most important event of our lifetime, casting a shadow over all our futures," Greengrass said. "It's vital for cinema to begin the task of exploring what it meant, and what it continues to mean today. Hopefully, 'Flight 93' will be a worthy contribution to that process."

A dedicated Web site will launch Wednesday including a forum for debate and a memorial to the passengers and crew of UA93. The pic has a large ensemble cast of American actors, but the producers are not identifying any starring roles out of respect for all those who died.

Tim Bevan, Eric Fellner and Lloyd Levin are producing the movie, which is scheduled for U.S. release April 28.