The US justice department has released the first video of the plane crashing into the Pentagon on 11 September 2001.
American Airlines Flight 77 slammed into the US military headquarters, killing 184 people, after it was hijacked as part of an al-Qaeda plot.
The release of the video, taken from a Pentagon security camera, comes after a Freedom of Information Act request by legal watchdog Judicial Watch.
The group said it hoped to dispel conspiracy theories about the crash.
"Finally, we hope that this video will put to rest the conspiracy theories involving American Airlines Flight 77," president Tom Fitton said.
Some theorists have suggested the aircraft was shot down in flight, and that the Pentagon was struck by a missile.
Stills released in 2002 showed the moment the hijacked plane crashed into the Pentagon, killing 125 people in the building and 59 passengers and crew.
WHAT IS JUDICIAL WATCH?
Describes itself as a conservative, non-partisan educational foundation
Has repeatedly sued US government agencies to obtain information
Forced release of documents on subjects ranging from RU-486 "abortion pill" to lobbyist visits to White House
Special focus on monitoring illegal immigration and Hillary Clinton fundraising
The crash came shortly after two other hijacked airlines were flown into the twin towers at the World Trade Center in New York.
Judicial Watch filed the freedom of information request in 2004, but the Pentagon refused to release the video because it was part of the investigation involving al-Qaeda plotter Zacarias Moussaoui, the group said.
This February the group sued the government over its refusal, saying there was "no legal basis" for it.
Washington agreed to release the video after a court sentenced Moussaoui to life in jail for his role in 9/11 earlier this month, a Judicial Watch spokeswoman told the BBC.
The group said it fought to obtain the video because "it was very important to complete the public record" on the attacks.
French author Thierry Meyssan alleged that Flight 77 did not crash into the Pentagon and suggested a truck bomb or missile caused the damage.
Such views are challenged by witness testimony at the scene of an aircraft fitting the description of Flight 77 crashing into the site.
Correspondents say the release of the video is not unhelpful to the Bush administration.
The footage serves as a graphic reminder of why the US launched its global "war on terror" and is still fighting it, the BBC's Adam Brookes in Washington says.