Completed in 1973, an exhibit called the World Trade Center, which is supposed to represent "world peace through trade", were two of the tallest buildings in the world. 9/11 was arguably the best security advance in America. It brought new laws, ways of travel, government agencies and anti-terrorism programs. David Rockefeller, grandson of John D. Rockefeller, decided to undertake the decision to build the World Trade Centers ("World"). Construction officially began in February 1967 ("World"). Traditional designs at the time were stacked glass and steel box constructions, and they knew they wanted the building to be different, so they used two hollow tubes supported by closely spaced steel columns encased in aluminum (“World”). Floor trusses connected the external steel lattice with the central steel core of the building. The "skin" of the building had to be strong to support the internal columns ("World"). To put the steel frame together, they used Australian-made ('World') 'kangaroo' cranes. They are self-powered cranes powered by diesel engines that rise as buildings grow ("World"). More than two hundred thousand pieces of steel were fabricated across the country to construct the buildings ("World"). The end of construction was April 1973 ("World"). The towers represented how powerful and wealthy America was. The day of the attacks was a normal day for citizens. Then, in an instant, everything changed. 19 militants of the Islamic extremist group al-Qaeda decided to implement a plan to attack America (“September 11 attacks”). At approximately 5:45 a.m., the hijackers passed security clearances and, a few hours later, took off on Flight 11 ("September 11 Interactive Timeline... half of document... to America, but America has gained information on how to protect the United States. America has become stronger and more powerful, so if something like this happens again, America will be ready to defend the nation at all costs .com A&E Television Networks. Web. May 21, 2014. “Information on the September 11, 2014 National Memorial.” Web. May 21, 2014. France-Presse, Agence responsibility for the September 11 attacks, says Son-in-Law ." Inquirer.net March 20, 2014. Web May 21, 2014. "Interactive Timeline of 9/11." Museum. 2014. Web. May 21, 2014. Villemez, Jason. "9/11 to Today: Ways We've Changed. " PBS September 14, 2011. Web May 21, 2014. "World Trade Center." A&E Television Networks and Web. May 20 2014.