9/11 25th anniversary ceremony: Families gather at WTC, Pentagon, Shanksville

Federal authorities are warning that foreign terrorist organizations "continue to encourage attacks in the United States and have released 9/11-related propaganda in advance of previous anniversaries of the 9/11 attacks," according to a bulletin reviewed by ABC News.

The bulletin says terrorist organizations continue to encourage attacks.

According to the bulletin, the FBI is concerned that terrorism actors in the U.S. may be inspired to conduct violence or engage in hoax threats in response to the 25th anniversary of the 9/11 attacks, and those actors have showed willingness to act with little to no warning.

Messaging by foreign terrorist organizations "in combination with ongoing conflicts in the Middle East could heighten existing grievances against the United States and contribute to mobilization to violence," according to the document.

Earlier Thursday, a Department of Homeland Security spokesperson said the U.S. is facing "a heightened and increasingly complex terrorism threat environment," adding that the environment has "changed significantly since the attacks of September 11, 2001."

"Before 9/11, the primary concern came from centralized international terrorist organizations. Today, those groups remain a threat, but the landscape is broader and more dispersed," according to a DHS spokesperson. "Individuals and small cells now plot attacks independently, often motivated by a range of ideological and political goals.

Foreign terrorist organizations, such as ISIS and al-Qaeda continue to seek ways to carry out or inspire attacks in the U.S., the spokesperson said.

"At the same time, domestic terrorists continue to plan acts of violence, often driven by anti-government views and a desire to destabilize society," the spokesperson added.

The Department of Homeland Security was born out of the 9/11 attacks with the goal of putting agencies who protect the homeland under one roof.

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For the first time, an additional bell and moment of silence will honor first responders, recovery workers, residents and survivors who have died from 9/11-related illnesses.

The new observance will give those who died in the years following the attacks their own moment in history as New York City marks 25 years since 9/11.

The FDNY suffered the most deaths of any first responder group at Ground Zero. Since 2001, the department has lost more than 600 members to 9/11-related diseases, surpassing the 343 firefighters who died on the day of the attacks.

FDNY Commissioner Lillian Bonsignore was at the EMS Academy in Queens when the planes struck the World Trade Center. She quickly rushed to Lower Manhattan and says that while 25 years have passed, New York's bravest continue to deal with the aftermath.

"That day has continued for 25 years for us," Bonsignore said. "We've lost 600 additional members over the years. Not just firefighters. EMTs and all those down there trying to help."

Bonsignore said getting care early helped the department gather information and provide treatment to those affected.

The somber milestone comes just days after New York City released thousands of pages of post-9/11 air quality documents suggesting officials at the time downplayed the health risks of the air quality.

Bonsignore said many of the deaths could have been avoided.

Americans are commemorating the 25th anniversary of 9/11, remembering the shock, the dead and the sweeping consequences of the al-Qaida attacks on symbols of U.S. might and prosperity.

Friday's milestone anniversary will bring memorial rituals - wreath-laying, flag-flying, moments of silence, vigils and readings of victims' names - and volunteer projects in tribute to a day that about half of U.S. adults remember as the most historic event of their lifetime, according to a new poll.

Traditionally, there are no political speeches at the New York commemoration, where relatives read all the victims' names, punctuated with moments of silence at key points in the attack timeline. For the first time, this year's ceremony will add a silent moment to honor people who were exposed to the clouds of toxic dust released by the twin towers' fiery collapse and have since died of cancer and other illnesses. Questions remain about how many people developed health problems linked to the dust

A massive security operation is underway in Lower Manhattan Friday as New York City marks 25 years since the September 11 attacks.

The NYPD will secure the 9/11 Memorial site with fencing and counterterrorism police as families and others head toward the reflecting pools for the remembrance ceremony.

Starting at 6 a.m., there will be no traffic south of Canal Street to Battery Place, from Broadway to West Street. The Hugh Carey Tunnel will remain open, along with southbound Broadway, but all trucks will be banned.

Intelligence officials say the terrorism landscape has changed in the years since the attacks.

"The threat is quite different from where it was 25 years ago. It's decentralized geographically with the rise of Al-Qaeda and ISIS affiliates globally," NYPD Deputy Commissioner of Intelligence and Counterterrorism Rebecca Weiner said.

Throughout the morning, several moments of silence will mark key moments from Sept. 11, 2001.

The first will be observed at 8:46 a.m., when Flight 11 struck the North Tower, followed by another at 9:03 a.m., when Flight 175 struck the South Tower. Additional moments of silence will mark the attack on the Pentagon and the crash of Flight 93 in Shanksville, Pennsylvania.

For the first time since the attacks, an additional bell and moment of silence will honor first responders, recovery workers, survivors and residents who have died from 9/11-related illnesses.

A number of dignitaries are also expected to join victims' families in New York City, including Mayor Zohran Mamdani, Vice President JD Vance and former Presidents Joe Biden, Barack Obama, George W. Bush and Bill Clinton.

President Donald Trump is expected to attend a ceremony at the Pentagon commemorating the lives lost in the attacks.

The quarter-century milestone will bring together those who were in Lower Manhattan that day, families who lost loved ones and a new generation learning about the attacks as the nation continues its vow to never forget.