9/11 ceremony live updates: Families to gather at WTC, Pentagon, Shanksville to mark 25 years

WABC-TV Eyewitness News anchor Michelle Charlesworth was the first reporter to arrive at the scene of the 9/11 attacks 25 years ago. She shared her reflections with viewers.

Michelle Charlesworth reflects on reporting from Ground Zero on Sept. 11

Fellow Eyewitness News reporter Nina Pineda arrived a short time later and described obtaining video from a man on the street with a camcorder. His video showed the first plane hitting the towers. She shared her reflections of that day.

Nina Pineda shares her story on Eyewitness News This Morning.

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