NEWS9/11 rememberedA second hijacked commercial plane approaches the burning World Trade Center shortly before crashing into the landmark skyscraper on Sept. 11, 2001 in New York. In the most devastating terrorist onslaughts ever waged against the United States, knife-wielding hijackers crashed two airliners into the landmark, toppling its twin 110-story towers. Commercial airliners were also flown into the Pentagon and crashed into a field in Pennsylvania. 2016 marks the 15th anniversary of the attacks.Seth McAllister, Getty ImagesA jet airliner is lined up on one of the World Trade Center towers in New York on Tuesday, Sept. 11, 2001.Carmden Taylor, APPeople in front of New York's St. Patrick's Cathedral react with horror as they look down Fifth Avenue toward the World Trade Center towers after planes crashed into their upper floors Tuesday morning, Sept. 11, 2001. Explosions and fires left gaping holes in the 110-story buildings, which eventually collapsed.Marty Lederhandler, APTwo women hold each other as they watch the World Trade Center burn following a terrorist attack on the twin skyscrapers in New York Tuesday, Sept. 11, 2001. Terrorists crashed two planes into the World Trade Center and the twin 110-story towers collapsed Tuesday morning.Ernesto Mora, APHijacked United Airlines Flight 175 from Boston crashes into the south tower of the World Trade Center and explodes at 9:03 a.m. on Sept. 11, 2001 in New York City.Spencer Platt, Getty ImagesChief of Staff Andy Card whispers to President George W. Bush that planes have crashed into the World Trade Center. Bush was visiting the Emma E. Booker Elementary School in Sarasota, Fla.Doug Mills, APFlorida Lt. Gov. Frank Brogan, left, and President Bush observe a moment of silence on the morning of Sept. 11, 2001 in Sarasota, Fla., for victims of the terrorist attack at the World Trade Center in New York.Chris O'Meara, APSmoke billows from the towers of the World Trade Center in New York, Tuesday, Sept. 11, 2001.STF, APEngine 28 firefighter Mike Kehoe, from Staten Island, assists in the evacuation effort in a stairwell of Tower One Tuesday, Sept. 11, 2001 during the attacks on the World Trade Center towers in New York. These pictures were shot by John Labriola, who had an office on the 71st floor of the building. He escaped with no injuries. Fellow firefighter Bobby Annunziato at Engine 28 in Manhattan said Kehoe escaped before the towers collapsed.John Labriola, APRescue workers carry fatally injured Father Mychal Judge, the New York City Fire Department chaplain, from one of the World Trade Center towers in New York City, early on Sept. 11, 2001.Shannon Stapleton, ReutersPlumes of smoke pour from the World Trade Center buildings in New York, Tuesday, Sept. 11, 2001, in this photo taken from the Brooklyn borough.Mark D. Phillips, APThe south tower of the World Trade Center, left, begins to collapse.Gulnara Samoilova, APPeople run from the collapsing World Trade Center Tower on Tuesday, Sept. 11, 2001.Suzanne Plunkett, APThe World Trade Center tower two crumbles to the ground after a fire early on Sept. 11, 2001.Jeff Christensen, ReutersPeople run as the North Tower of the World Trade Center collapses after being hit by an airliner on Sept.11, 2001.Jose Jimenez, Primera HoraA woman covered in dust takes refuge in an office building after the top of one of the World Trade Center towers collapsed. Marcy Borders, then 28, who became known as the "dust lady" after the photo was seen around the world, died after a battle with cancer at the age of 42.Stan Honda, Getty ImagesA woman reacts to a third explosion, possibly the collapse of the World Trade Center towers, while observing from the Brooklyn Promenade, which provides a view of the Manhattan skyline, Tuesday, Sept. 11, 2001, in New York.Kathy Willens, APWomen walk near Wall Street shortly after the collapse of the World Trade Center.Justin Lane, NYTNew York Firefighter John Cleary wipes soot from his face while taking a break from rescue work at the World Trade Center in New York on Sept. 12, 2001. Clearly helped in the rescue of two trapped Port Authority workers, pulling the first from the rubble at one this morning and helping the second to safety about four hours later.Brad Rickerby, ReutersJoseph Curry, New York Fire Department deputy chief, barks orders as firefighters and rescue teams search for survivors through the rubble and debris of the World Trade Center.Preston Keres, USNColleagues huddle together following the collapse of the World Trade Center towers.Krista Niles, NYTA firefighter rests her chin on a helmet during a break as she watches the cleanup of the World Trade Center attacks on Thursday, Sept. 13, 2001 in New York.Suzanne Plunkett, APA law enforcement officer reacts after the first tower of the World Trade Center collapses.Shannon Stapleton, ReutersFirefighters and rescue personnel tend to the injured amid the ruins of the World Trade Center shortly after its collapse on Sept. 11, 2001.Justin Lane, NYTEmergency workers help a women injured in the terrorist attack on the World Trade Center in New York.Gulnara Samiolava, APPeople flee the scene near New York's World Trade Center on Tuesday, Sept. 11, 2001. In a horrific sequence of destruction, terrorists crashed two airliners into the World Trade Center in a coordinated series of attacks that brought down the twin 110-story towers.Diane Bondareff, APLisa Bianco, left, comforts two women near New York's World Trade Center after terrorists crashed two planes into the landmark buildings.Matt Moyer, APPeople flee lower Manhattan across the Brooklyn Bridge in New York, Tuesday, Sept. 11, 2001, following a terrorist attack on the World Trade Center.Daniel Shanken, APA helicopter flies over the burning Pentagon Tuesday, Sept. 11, 2001 after it was hit by a hijacked commercial airliner.Tom Horan, APThe Pentagon is on fire after a hijacked aircraft crashes into it on Sept.11, 2001.Hyungwon Kang, ReutersSmoke and flames rise over the Pentagon the evening of Sept. 11, 2001 following a terrorist crash of a commercial airliner.Ho, ReutersFire fighters and rescue workers unfurl a large U.S. flag near the damaged area of the Pentagon Building at the U.S. Military Headquarters outside of Washington, September 12, 2001.Larry Downing, REUTERSFirefighters and emergency personnel investigate the scene of the crash of a United Airlines Boeing 757 with at least 45 passengers Tuesday morning, Sept. 11, 2001 near Shanksville, Pa., Somerset County. Radar showed the San Francisco-bound Boeing 757 from Newark, N.J., had nearly reached Cleveland when it made a sharp left turn and headed back toward Pennsylvania, crashing in a grassy field edged by woods about 80 miles southeast of Pittsburgh. It became known later that passengers aboard the doomed jet fought back against the hijackers, diverting them from another target.David Lloyd, APU.S. Federal Investigators search for clues near the damaged area of the Pentagon Building as firefighters and rescue workers continue to battle smoke at the U.S. Military Headquarters outside of Washington, D.C., on Sept. 12, 2001.Larry Downing, ReutersSmoke and ash from the destroyed World Trade Center rise over the southern end of New York City's Manhattan behind the Statue of Liberty as seen from Jersey City, New Jersey, Sept. 12, 2001. Terrorists hijacked two commercial airliners and crashed them into each of the towers of the Trade Center, collapsing them.Tannen Maury, AFP/Getty ImagesEarly morning light hits the smoke and wreckage of the World Trade Center on Sept. 13, 2001 in New York City, two days after the twin towers were destroyed when hit by two hijacked passenger jets.Chris Hondros, Getty ImagesPolice officers stand guard near the site of the World Trade Center in New York, Wednesday, Sept. 12, 2001. In the most devastating terrorist onslaught ever waged against the United States, knife-wielding hijackers crashed two airliners into the World Trade Center on Tuesday, eventually toppling its twin 110-story towers.Beth A. Keiser, APMourners outside the firehouse for Fire Company 226 on State Street in Brooklyn Heights on Sept.13, 2001. Four of the five firefighters who went to the World Trade Center on Sept. 11 did not return.Nancy Siesel, NYTDays after the attack on the World Trade Center, Michele Defazio held up a poster of her missing husband Jason Defazio who worked for Cantor Fitzgerald on the 104th floor of One World Trade Center. Defazio had gone to the Park Avenue Armory to file a missing person's report in the hopes of finding any information about her husband. The couple had only been married two-and-a-half months.Krista Niles, NYTMary Ortele of Brooklyn, N.Y., holds a picture of herself with her missing husband, Peter, as she is hugged by her mother, Kathy Adlun, at the 69th Regiment Armory in New York Saturday, Sept. 15, 2001. Ortele's husband had been missing since Tuesday's terrorist attack against the World Trade Center.Matt Moyer, APCurtis Larson holds three photos of his son, Jude Larson, Tuesday afternoon, Sept. 11, 2001, in Lahaina, Hawaii. His 31-year-old son and Jude's pregnant wife, Natalie, who lived in Los Angeles were among the passengers on a United Airlines flight that was hijacked and crashed into one of the World Trade Center towers in New York City. Jude Larson is shown as an 8-year-old in the bottom photo. He mugged for the camera with his uncle in the photo at right.Matthew Thayer, APMuslim Lena Beck, foreground, joins in a candlelight vigil at McKenzie Park in Panama City, Fla., on Wednesday, Sept. 12, 2001, to remember those killed in Tuesday's terrorist attacks in Washington, New York and Pennsylvania.Terry Barner, APRobert and Debi Elston of Flint embrace during a candlelight vigil at the Flint Cultural Center Wednesday night, Sept. 12, 2001. The vigil offered words from clergy as well as the singing of spiritual and patriotic songs in the wake of the terrorist attacks on Tuesday.Bruce Edwards, APU.S. Muslim Imam Sayed Moustafa Al-Qazwini lights a candle, joined by leaders of other religions, on Sept.13, 2001 in Pasadena, Calif, at an Interfaith Memorial Service for victims of the Sept. 11 terrorist attacks on New York and Washington, D.C.Lucy Nicholson, Getty ImagesStudents light candles at an altar after a memorial service to remember the victims of the U.S. terrorist attacks at the cathedral in Dresden, eastern Germany, Thursday, Sept. 13, 2001. The attacks prompted an outpouring of sympathy and solidarity for the United States around the world.Matthias Reitschel, APSaudi terrorist Osama bin Laden, shown in Afghanistan, was the mastermind of the attacks carried out by Al-Qaida. He was killed in a U.S. raid ordered by President Barack Obama on May 2, 2011.APPresident George W. Bush puts his arm around firefighter Bob Beckwith while standing in front of the World Trade Center in New York on Sept. 14, 2001 amid rescue efforts.Doug Mills, APEmergency crews conduct search and rescue attempts, descending deep into the rubble and debris of the World Trade Centers in New York City in the area known as Ground Zero.Jim Watson, USNBrooklyn firefighters George Johnson, left, of ladder 157, Dan McWilliams, center, of ladder 157, and Billy Eisengrein, right, of Rescue 2, raise a flag at the World Trade Center in New York on Sept. 11, 2001.Thomas E. Franklin, AP