42. A second aircraft approaches the World Trade Center just prior to hitting the south tower. Within a span of 18 minutes, two planes had crashed into each of the twin towers of the World Trade Center in Manhattan in what President Bush called "a national tragedy and an apparent act of terrorism against our country.“ Kelly Guenther for The New York Times

43. A fireball erupts from one of the towers of the World Trade Center after the second plane hit. In the background, smoke billows from the other tower that was struck minutes earlier. The Brooklyn Bridge is in the foreground.

44. From about half a mile north, near Canal Street, people's expressions and gestures bore witness to the sight they beheld: the collapse of 2 World Trade Center. Angel Franco/The New York Times

45. As people fled across the Brooklyn Bridge from Lower Manhattan, some of them betrayed their curiosity about Joseph Sylvester, a World Financial Center employee who had obviously been close to ground zero. Andrea Mohin/The New York Times

46. A woman stands alone in the middle of a gaping hole in the World Trade Center, surrounded by smoke and flames, with nowhere to go and no way to be rescued. Brian Manning

47. A victim from the World Trade Center attack is treated by emergency medical personnel on the east side of Church Street across from 5 World Trade Center. She was evacuated before the buildings collapsed. Ruth Fremson/The New York Times

48. By the time the twin towers collapsed that morning, more than 2,900 people were dead, lower Manhattan was covered in ash-laden debris and the lot where two of the world's tallest buildings had stood for more than a generation had a new name, ground zero. Chang W. Lee/The New York Times

49. Fire fighters and paramedics assist the injured in the immediate aftermath of the twin towers' collapse. Justin Lane for The New York Times

50. An exhausted New York City police officer, Richard Adamiak, 26, ducked into the Stage Door Deli on Vesey Street across from the World Trade Center Plaza after the collapse of the first tower -- and shortly before the second tower came down. Ruth Fremson/The New York Times

51. Rescue workers and medical personnel continued their efforts in the middle of a vast, nightmarish landscape on West Street 24 hours after the collapse of the World Trade Center Towers. Ruth Fremson/The New York Times

52. A firefighter of Ladder 21 is embraced by fellow firefighters after being told a family member, who is also a firefighter that responded to the World Trade Center attacks, could not be located after the buildings collapsed. Krista Niles for The New York Times

53. Two women made their way through the ash-covered streets of lower Manhattan in the immediate aftermath of the crumbling of the World Trade Center towers. Justin Lane for The New York Times

54. The End