Nineteen terrorists hijacked four passenger airplanes for the attacks, and according to media sources, the whole plan was masterminded by the group's leader Osama bin Laden. Two planes, United Airlines Flight 175 and American Airlines Flight 11 crashed into the World Trade Center Towers. American Airlines Flight 77 crashed into the western side of the Pentagon, which is the Headquarters of the Department of Defense. The fourth flight, United Airlines Flight 93, was initially directed towards Washington D.C., but went down in a meadow at Shanksville in Pennsylvania.
The terrorist attacks were condemned by the nation shortly after they took place, along with other countries showing their support. Many middle-eastern countries too denounced the act, including Afghanistan. Hate crimes against Middle-Eastern origin and Asian-origin people began to rise a lot, like murders, thievery, arson, etc., after September 11. Several bank accounts thought to belong to terrorists were closed down.
On the International front, USA strengthened its stand against terrorism, and continued deploying troops in Afghanistan to combat the terrorists. This can be said to be one of the direct effects of the September 11 attacks, as the British also joined forces with the US army in the War of Afghanistan. The war that was waged against Iraq can also be considered to be one of the effects of these attacks.
Regarding health effects, the toxins that arose because of the crashes were responsible for many deaths due to asphyxiation, and air quality in the immediate surroundings worsened a lot. Many pregnant woman gave birth to premature babies, and some were even stillborn. On an average, more than 15,000 individuals are known to have developed a variety of illnesses due to these attacks.
In case of effects on the economy, the stock markets were closed till 17th September, and the losses were pegged at more than USD 1 trillion. More than 4,00,000 jobs were lost, and New York City's GDP decreased considerably. The entire American airspace was closed for many days, and after it started, the traffic situation worsened drastically.