9/11: What happened on September 11 2001?

On the morning of September 11 2001, 14-year-old Walter was starting his first day of high school in New York.

It's a day he'll never forget, because an attack in his city changed the world forever.

"It was a sunny September morning, it was Tuesday and I was in English class," Walter says.

"I remember one of my classmates ran in and said; 'the World Trade Center, something happened', and that there had been fires or a bomb.

"As the day progressed, that's when you really realised what was happening."

What happened on 9/11?

At 08:46am office workers and tourists were getting ready to start the day in New York City when a hijacked plane was deliberately flown into the north tower of the World Trade Center.

Seventeen minutes later, a second hijacked plane was crashed into the south tower.

The two 110-floor buildings, the tallest skyscrapers in New York, were now on fire as hundreds of emergency service workers rushed to the towers to help those inside.

A police officer looks up at the north tower of the World Trade Center

At the time of the attack, Walter's aunt worked in another World Trade Center building next to the Twin Towers.

"My aunt worked at World Trade [building] number seven, so I knew she was on her way to work," Walter remembers.

"I remember calling my grandmother and asking; 'Do you know where aunt Pam is?' And her saying; 'I'm watching the news, I don't know where she is...'"

The 9/11 Pentagon attack

The US president in 2001, George W Bush was visiting a school in Florida when the attack happened. From the school he announced that the events in New York City were an "apparent terrorist attack on our country."

At 09:37, moments after the president's announcement, a third hijacked plane was deliberately crashed into the side of The Pentagon, the US defence headquarters near the capital city, Washington DC.

A fourth plane, thought to be targeting the US Capitol Building, also in Washington DC, crashed in a field in Pennsylvania after passengers fought back against the hijackers.

The twin towers collapse

Less than two hours after being hit by the planes, the twin towers of the World Trade Center in New York collapsed - one after the other - leaving massive clouds of dust and debris.

Walter's aunt managed to escape lower Manhattan, the area of city where the towers had fallen.

"She didn't get home until late, I remember being in the house waiting with my cousin and my grandmother and her walking through the door," Walter says.

"She had got on the back of somebody's truck to get home" he added, explaining that the public transport in New York had been diverted and many bridges and tunnels were closed to traffic, other than emergency vehicles.

In total, 2,977 people (not including the 19 plane hijackers) died in the attacks and thousands were injured.

Many firefighters and police officers have developed illnesses since 9/11, having worked to save people in toxic dust, smoke and debris caused by the falling buildings.

"I didn't know what people were capable of"

A dust covered street near to where the Twin Towers collapsed on September 11 2001

The towers were an iconic part of the city and were seen in lots of movies and TV shows set in New York before 2001.

The attack was designed to shock the world, with millions of people watching the events unfold live on the news, including in other parts of the United States.

In California, towards the west coast of America, it was 07:30 in the morning when the second tower collapsed.

Eleven-year-old Carrie had just got on the bus to school.

"The second tower had just fallen and everyone was really confused," she says.

"I remember my teacher at the time, she was very sad.

"Instead of doing subjects at school we just sat and talked about what was going on and it was like; 'the United States was attacked, New York, The Pentagon'," Carrie explained.

"We had to have a conversation that 'people do terrible things, and we don't really know what's going to happen from here'."

Now in her 20s, Rachel was eight-years-old at the time of the attacks and lived in Maine, a US state north of New York City.

"I was just watching the TV and it kept showing the attack over and over again," she says gesturing to explain how the news repeated footage of the planes hitting the buildings.

"I'm like eight, I didn't know what people were capable of.

"I was very anxious and very frightened."

Why was America attacked on 9/11?

The group responsible for 9/11 was identified as al-Qaeda, an Islamist extremist group who planned the attacks from Afghanistan.

Led by a man called Osama bin Laden, al-Qaeda blamed America for many of the world's problems.

The group started their plan, known as the 'planes attack' in 1998.

By the year 2000 they had made their way into the United States.

Nineteen people carried out the plane hijackings. Each group included someone who had received pilot training. The training had been carried out at flying schools in the US itself.

US security officials knew the group were targeting the country but did not know where and were taken by surprise when the well organised attack happened.

How did America respond to the 9/11 attacks?

President George W Bush speaks to firefighters and rescue workers at the site of the collapsed World Trade Center

The attacks caused international outrage, and led to the US President George W Bush declaring a "war on terror".

In October 2001, America led an invasion of Afghanistan - supported by other countries including Britain.

Afghanistan was governed by the Taliban who had an alliance with al-Qaeda.

The US, UK and other countries' mission in Afghanistan was to destroy al-Qaeda, hunt down bin Laden and remove the Taliban from power.

Although America and its allies succeeded in removing the Taliban government, it was ten years later that the US found and killed Osama bin Laden.

The military involvement in Afghanistan continued for several years, becoming the longest war in America's history.

It's only this year, 20 years after 9/11, that the final British and American troops left the country as the Taliban took control of Afghanistan again.

"The events have altered the world and especially the United States of America, you look at the news today of Afghanistan and understand how we got there and why we were there," says Walter.

"It shaped a lot of people, I have classmates who I was in high school with who became a marine or went into the army, specifically because of this, because of what happened on September 11."

9/11 changed America and the world

Safer flying

Flight safety was tightened and improved in the years following 9/11.

Before September 11, children could go up to visit the plane's cockpit and meet the pilots. But that all stopped after the attacks and cockpit doors were locked and bullet-proofed.

The security in airports around the world also became much stricter, including in the UK.

Islamophobia

Following the September 11 attacks, the numbers of Islamophobic hate crimes against Muslims increased in both the US and in the UK.

Islamophobia is when Muslims are victims of abusive words or violence just because of their religion.

Pictured in 2010, New York City police put up barricades after protests for and against the building of a new mosque, just a few blocks away from where the twin towers collapsed

Remembering how the terrorist attacks changed America, Rachel recalls that during her childhood, she noticed how people had become more suspicious of others.

"Security in every shape and form was amped up everywhere. I felt like nobody trusted anyone."

"9/11 did happen, but it's not like those individuals represent an entire religion or culture. But that racism happens now.

"We have security for purposes like this, but we also have love and appreciation for other cultures because not everybody looks like you, thinks like you and not everybody who is different from you is bad."

New York after 9/11

How the New York skyline looked 20 years ago compared with today. A new One World Trade Centre building, also known as 'Freedom Tower' stands behind the footprints of the original buildings

Rebuilding work at the Pentagon took just under a year, with staff back in their offices by August 2002.

In New York, it took more than eight months to clean up "Ground Zero" - the name given to the site of the fallen Twin Towers.

A memorial and a museum now stands there, and new buildings have been built with a different design.

Today, the tallest building in New York City - One World Trade Center, or Freedom Tower - stands even higher (1,776ft (541m) than the original North Tower, which - at the top of its antenna - was 1,728ft tall.

Rachel, now lives and works in New York City, she explained what it's like to visit the memorial and to remember the firefighters, police officers and "the heroes who went down and were pulling people out of the rubble."

"You can still just feel this layer of like 'something happened here', but New York comes back from everything, you know, 'cause the whole world is in that city.

"It's like you're taking down New York, America, but people from the entire world are living there and the community is so strong there."

Two beams of light in the sky next to the New York City skyline, in tribute to the Twin Towers and the victims of 9/11