The 9/11 terror attacks are forever etched in the collective memory of the world.

On September 11, 2001, the world witnessed horrors hitherto unseen as the Twin Towers in New York City, USA, came crumbling down after two commercial flights crashed into them.

Another jetliner hit the Pentagon, and a fourth crashed in Pennsylvania.

Hindu from the ground, completely changed the way the world looked at terrorism.

Its scars are still visible 25 years later, in the form of the 9/11 Memorial and Museum and the accounts of many who lost loved ones.

At its core, the 9/11 terror attacks involved four planes that were hijacked by terrorists.

Their intended targets were the North and South Towers of the World Trade Center in New York and the Pentagon.

The target of the fourth flight has been speculated widely, with some saying that even the White House could have been the final destination of that fateful aircraft.

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The four flights that were involved were American Airlines Flight 11, United Airlines Flight 175, American Airlines Flight 77, and United Airlines Flight 93.

The world watched in horror as the city descended into chaos and smoke spread across New York with people jumping from the Twin Towers.

A jet airliner is lined up on one of the World Trade Center towers in New York, September 11, 2001 | Photo 9/11 victims Nearly 3,000 people died on 9/11, according to the FBI, which adds that the victims of the attack were from New York, Washington, D.C. and 27 other states of the country.

The US Army lost 75 personnel in the attack.

More than 400 first responders, including 60 law enforcement officers, were killed, and nearly 3,000 children lost a parent.

Who was responsible?

According to the FBI, the 19 men who hijacked and crashed the four planes were all trained by terrorist organisation al-Qaeda.

Three of the suspected pilots - Mohamed Atta, Marwan Al-Shehhi, and Zaid Jarrah - were part of the terror cell in Hamburg, Germany.

Fifteen of the hijackers came from Saudi Arabia, two from the United Arab Emirates or UAE, one from Egypt, and one from Lebanon.

The FBI adds that the hijackers began entering the US in January 2000, and all nineteen of them were in the country by early July 2001, three months before that tragic day.

The Hindu on the day after, with images of a crumbling World Trade Centre dominating the coverage.

The Hindu’s then foreign correspondent Sridhar Krishnaswami, while reporting on the immediate aftermath, wrote that “what has stunned intelligence and law enforcement agencies, as also the common man, is the kind of coordination and precision the terrorist acts had been executed.” Describing the horrifying scenes that he witnessed, Mr.

Krishnaswami wrote about how smoke and shock spread all over New York and that “the first impression here and elsewhere is where intelligence went wrong and some are already talking about colossal intelligence failure.” 9/11 attacks: Photos that defined the day of terror An American flag flies near the base of the destroyed world trade center in new york, September 11, 2001.

Planes crashed into each of the two towers, causing them to collapse.

Emergency personnel work in the debris of the World Trade Centre in New York, Wednesday, September 12, 2001.

Rescue workers dug for bodies in mountains of rubble as the city struggled to recover from an airborne attack on the World Trade Centre on Tuesday that shut down the nation’s financial capital and created a new skyline etched in terror.

Rescue workers and vehicles are deployed near the site of the World Trade Centre in New York, Wednesday, September 12, 2001.

Combo TV images show a tower of the World Trade Centre collapsing after two planes crashed into the complex on September 11, 2001, destroying New York’s mighty twin towers.

The 9/11 attack's "falling man" was one of the most iconic and haunting images, depicting a person falling headfirst from the north tower of New York’s World Trade Centre on Tuesday, September 11, 2001.

Smoke, flames and debris erupt from one of the World Trade Centre towers as a plane strikes it on September 11, 2001.

The first tower was already burning following a terror attack minutes earlier.

Terrorists crashed planes into the two buildings, causing both towers to collapse.

The south tower of the World Trade Centre bursts into flames after being struck by United Airlines Flight 175 in New York on September 11, 2001.

With the Brooklyn Bridge in the foreground, a plane explodes after hitting the second tower of the World Trade Centre as the other tower burns, in New York, September 11, 2001.

Both towers of the complex collapsed after hijacked planes hit them.

The remaining tower of New York’s World Trade Centre, Tower 2, dissolves in a cloud of dust and debris about a half hour after the first Twin Tower collapsed September 11, 2001.

The pictures were taken from across the Hudson River in Jersey City, New Jersey.

Firemen work around the World Tade Center after both towers collapsed in New York on September 11, 2001.

The kamikaze airplane attacks that destroyed New York’s World Trade Centre and damaged the Pentagon on Tuesday were unprecedented in the history of civil aviation, security experts said.

In this September 11, 2001 photo made by the New York City Police Department and provided by ABC News, smoke and ash engulf lower Manhattan after terrorists flew two airliners into the World trade Centre towers.

A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airliner crashed into the building in this September 11, 2001, file photo.

The World Trade Centre in New York City burns as a police helicopter flies nearby early September 11, 2001.

Both towers were hit by planes crashing into the building.

Shortly after this photo was taken, both towers fell.

Firefighters and emergency personnel remove debris from the site of the World Trade Centre towers’ collapse in New York, September 13, 2001.

An original piece of the Twin Towers, known as a "trident," is lowered into its permanent location at the 9/11 Memorial Museum at ground zero in New York, Tuesday, September 7, 2010.

Behind the "trident" is One World Trade Center, still under construction.

Firefighters raise a flag late in the afternoon on Tuesday, September 11, 2001, in the wreckage of the World Trade Centre towers in New York.

In the most devastating terrorist onslaught ever waged against the United States, knife-wielding hijackers crashed two airliners into the World Trade Centre, toppling its twin 110-story towers.

Photos of victims of the attacks of September 11, and messages from their loved ones, are shown at a news conference on September 10, 2012 in New York.

On the eve of the September 11 anniversary, the faces and recorded voices of those who died were unveiled as part of the future 9/11 Memorial Museum.

A family member of an Israeli victim of the September 11 attacks, points to the victim's name, engraved on a monument, during a memorial event at the 9/11 Living Memorial Plaza in Jerusalem on September 11, 2016.

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