9/11 rescue dogs' best friends were the people who kept dogs safe at Ground Zero .

Firefighters at a New Jersey gym walked up 110 flights to honor those who lost their lives in the 9/11 attacks.

A new study says up to half of the $14 trillion spent by the Pentagon since 9/11 went to for-profit defense contractors

Melanie Elturk, like many Muslim women who wear a hijab, noticed a stark change in how she was perceived after the terrorist attacks on the World Trade Center in 2001.

The Biden administration began to release newly declassified documents on the Sept. 11, 2001, investigation over the weekend.

Shannon Stapleton and Kayla Bergeron have bonded over tragedy.

Former President George W. Bush spoke at the Sept. 11 memorial at Shanksville, Pennsylvania, to mark the 20th anniversary of the attacks Saturday.

The document is from the investigation into possible connections between the 9/11 hijackers and the Saudi Arabian government.

New York City commemorated the 20th anniversary of 9/11 with the familiar 'Tribute in Light' beams. Many buildings across the city were also illuminated in blue.

The 9/11 attacks set off a wave of Islamophobia in America that many peaceful and patriotic Muslims say still reverberates years later.

President Joe Biden, first lady Jill Biden, Vice President Kamala Harris and second gentleman Douglas Emhoff participated in the ceremony to lay a wreath at the Pentagon.

President Joe Biden paid a visit to the Shanksville Volunteer Fire Company in Pennsylvania, which was first to respond to the Flight 93 crash on Sept. 11, 2001.

Bells rang for six moments of silence to mark the 20th anniversary of the Sept. 11 attacks and to honor the victims.

Americans marked 20 years since the 9/11 terrorist attacks in a series of events around the country.

Victims' families observed the pretrial proceedings this 20th anniversary.

Tributes are being held across the country to honor the lives lost on 9/11 and the first responders who rushed in to help.

President Joe Biden released a message to the nation focusing on the unity that followed the attacks

Shanksville, Pennsylvania, remembers an act of supreme heroism by passengers saving lives while sacrificing their own by taking United Flight 93 down.

More than 64,000 claims for assistance have been made by survivors and families to the Victims' Compensation Fund.

"Tunnel to Tower" founder Frank Siller, continues to inspire with his walk through the Brooklyn Battery Tunnel commemorating his firefighter brothers' run to the Towers.

Firefighter cyclists peddled from Santa Monica to New York, biking more than 3,200 miles in 40 days.

Three individuals directly impacted by the World Trade Center attacks on Sept. 11, 2001, look back on how their lives have changed since that infamous day 20 years ago.

Harris: "We are gathered here today...to honor the heroism that the 40 passengers and crew members showed in the face of grave terrorism."

The former president addressed the crowd at Shanksville, Pennsylvania, where a third hijacked plane, United Flight 93 crashed into a field, killing all 40 crew and passengers on Sept. 11, 2001.

ABC's Chief Global Affairs Correspondent Martha Raddatz takes a look at post-9/11 security concerns.

Oscar-winning filmmaker and host of the podcast Rumble Michael Moore joins us to reflect on Sept. 11.

On 20th anniversary of 9/11 attack, questions, anger, revenge and death yet linger

The terror attacks fueled anti-Muslim sentiment across the U.S. and the world and the effects are still visible today.

The passengers aboard United Flight 93 fought back against the plane’s hijackers before it crashed in Shanksville, Pennsylvania.

Forensic investigators identified two more victims using DNA evidence just this week.

Twenty years ago, 3,000 people went to work on a cloudless morning and never returned home.

"Unity is what makes us who we are," the president said, calling on Americans to come together ahead of the 9/11 anniversary while paying his tribute to the victims.

ABC News’ Linsey Davis speaks with the Honorable Suzan Johnson Cook, the NYPD chaplain on Sept. 11, 2001, about the messages she delivered on that day and in the immediate aftermath.

ABC News’ Linsey Davis speaks with four children of fallen firefighter Carl Asaro, who have all become firefighters themselves in the 20 years since the Sept. 11 attacks.

A look at what’s been built at the former site of the World Trade Center towers, and what remains unfinished 20 years after the Sept. 11 attacks.

ABC News’ Linsey Davis speaks with NYPD Chief of Counterterrorism Martine Materasso on how the city prepares against terror threats ahead of the 20th anniversary of Sept. 11.

Robert Atwood, who was born six months after his father died at ground zero, wears his father’s firefighter badge number on a chain.

For 20 years, Diane Sawyer has periodically gathered with a group of women who were pregnant when their partners died on Sept. 11. Their stories show the journey of grief amid raising a family.

For nearly 20 years, ABC News has followed the people forever linked to the terror attacks of Sept. 11, 2001. David Muir retraces the events of that fateful day.

Of the 15 people who left from Engine 54, Ladder 4 that day, none returned home.

Biden will attend three 9/11 memorials on the 20th anniversary of the attacks on Saturday.

The day before the 20th anniversary of the 9/11 terrorist attacks, a pretrial hearing against five accused orchestrators was held Friday with none of them present.

Firefighters reflect on a cross-country bike ride in honor or 9/11.

Joseph Reina grew up never knowing his father but carries his name.

Many mom’s pregnant when they learned their husbands would never come home again, Diane Sawyer brings us a preview of tonight’s special 20/20.

Residents of rural Stanford, New York, remember the fallen.

FDNY Chaplain shares words of hope on 20th anniversary of the 9/11 terrorist attacks.

Dr. Jen Ashton with answers to viewer’s medical questions.

Dr. Jen Ashton discusses the physical health issues 9/11 first responders face, including autoimmune diseases, lung problems and cancer.

First responder and survivor advocate John Feal on the 20th anniversary of 9/11.