15 years after Sept. 11, the questions that still remain in our minds

What happened on Sept. 11, 2001?

Nineteen men hijacked four fuel-laden U.S. commercial airplanes and crashed them into each tower of New York City’s World Trade Center complex, the Pentagon building in Washington and a field in Shanksville, Pa.

How many people died? Were injured?

The attacks caused a total of 2,996 deaths, making it the deadliest foreign attack ever on U.S. soil. It surpassed the 1941 attack on Pearl Harbor, which claimed 2,403 American lives. More than 6,000 others were injured in the 9/11 attacks.

Who was behind the attacks, and why? The terrorist group al-Qaeda coordinated and took credit for the attacks. An earlier declaration of holy war against the United States by al-Qaeda’s leader, Osama bin Laden, was seen as the main motivator for the hijackers. Fifteen of the 19 hijackers were from Saudi Arabia and the rest originated from the United Arab Emirates, Egypt and Lebanon. Some had lived in Europe and were able to assimilate in the USA.

What was the World Trade Center? What was it used for before 9/11?

The World Trade Center was a complex of seven buildings in Lower Manhattan used mostly as office and commercial space. At the time of their completion in the early 1970s, the landmark twin towers, standing at about 1,300 feet, were the tallest buildings in the world.

Were the 9/11 attacks the first time the World Trade Center was targeted?

No. In 1993, terrorists detonated a truck bomb in the underground garage beneath the World Trade Center complex. The explosion tore a hole seven stories up, killed six people and injured more than 1,000. But the towers remained standing. The FBI later arrested several Islamic terrorists responsible for the bombing.

Was America caught completely off guard by the 9/11 attacks, or was there intelligence suggesting an attack was coming?

There were several incidents leading up to the 9/11 attacks that hinted at a larger attack coming, including the 1993 World Trade Center bombing, the 1998 bombings of U.S. embassies in Kenya and Tanzania, and the 2000 attack on the USS Cole while harbored in Yemen. The CIA had even tracked some of the 9/11 hijackers into the United States as potential threats, but authorities never acted on that information.

How did the hijackers commandeer those planes? And how did the passengers aboard the flight that crashed in Shanksville, Pa., overpower them? Where was that plane headed?

The hijackers entered the cockpit of all four planes and killed or subdued the crew using box cutters and other rudimentary tools. Passengers aboard United Airlines Flight 93 revolted against the hijackers and managed to get into the cockpit before the hijackers drove the plane into the ground, killing everyone onboard. The plane was headed to Washington, D.C., where its likely target was either the White House or the Capitol.

Less than a month after the attacks, the U.S. and its allies invaded Afghanistan. Why?

Al-Qaeda, sheltered by the Taliban, an Islamic fundamentalist political movement that dominated Afghanistan from 1996 to 2001, quickly took credit for the 9/11 attacks. Then-President George W. Bush, with backing from Congress, ordered a full military invasion of Afghanistan to disrupt al-Qaeda and drive the Taliban out of power.

How did that lead America into war with Iraq? What was the role of “weapons of mass destruction?”

In the wake of the 9/11 attacks, the Bush administration alleged ties between al-Qaeda operatives and Iraq’s intelligence services. Bush officials also said Iraq had weapons of mass destruction, specifically mobile biological weapons labs. None were ever found following the invasion and occupation of Iraq.

How long did it take to find Osama bin Laden?

It took U.S. authorities nearly 10 years to track down and kill bin Laden, considered the architect of the 9/11 attacks, despite a $25 million bounty on his head. On May 2, 2011, a team of Navy SEALs raided his compound in Abbottabad, Pakistan, killing bin Laden and several of his bodyguards.

What’s at the site of the World Trade Center now? The Pentagon? Shanksville?

The World Trade Center site is now home to the National September 11 Memorial and Museum and the 1,776-foot-tall One World Trade Center. Other skyscrapers and buildings are planned for the site. At the Pentagon, there's an outdoor memorial at the site where American Airlines Flight 77 struck the building, and, inside, there's a memorial honoring victims of the attack. In Shanksville, Pa., a memorial embedded into an open field honors the passengers who brought down the hijacked United Airlines Flight 93. 

How has America changed since the 9/11 attacks?

The attacks greatly increased government focus on terrorism, not just in the U.S. but around the world. The U.S. Department of Homeland Security was created to coordinate efforts between security agencies, airports beefed up passenger screenings, and cooperation and intelligence gathering between nations sharply increased. “We now see terrorism as a top responsibility of presidents and our political leaders in a way we didn’t before 9/11,” said Daniel Byman, a national security professor at Georgetown University and senior fellow at the Brookings Institution.

Could a similar attack occur in America today?

Not impossible, but not likely. The improved intelligence sharing between U.S. security services and between nations, increased attention to terrorist groups and, most importantly, increased cooperation from U.S. Muslim communities make it much harder to pull off a terrorist attack of that scale, Byman said. In the 15 years since the 9/11 attacks, 94 people have died in jihadist terrorist attacks on U.S. soil — far less than those killed by car accidents, heart disease or accidental gun discharges. “All those things that led [the 9/11 attacks] to succeed would be much harder to do today,” he said.

More 9/11 stories: 

• 10 things you might have forgotten about 9/11
• 15 years after 9/11, lost unity: Our view
• Unforgettable images from September 2001
• Never Forget: 15 years later, 9/11 tattoos remain strong
• Why memorial tattoos are important
• 9/11 years later: Terror continues for many who survived the attacks
• 1,113 families still have no real confirmation of 9/11 deaths
• Newly published notes recount 9/11 aboard Air Force One: 'We're at war'
• Do tragedies like 9/11 have an expiration date?

Sources: The 9/11 Commission Report, Global Terrorism Database, Stastistic Brain, 9/11memorial.org, USA TODAY research