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Twenty Years Since 9/11: The Evolving Terrorist Threat to the U.S. Homeland

September 15, 2021
By Naureen Kabir

Saturday, September 11, 2021 marked the twentieth anniversary of the 911 terrorist attacks — the single deadliest terrorist attack on the U.S. homeland, perpetrated by al-Qaeda operatives and planned out of the group’s Taliban-enabled safe haven in Afghanistan. The twentieth anniversary of the attacks came at the heels of the Taliban takeover of Kabul and the U.S. withdrawal from Afghanistan after nearly two decades, which have fueled renewed questions about the threat posed by al Qaeda and other foreign terrorist organizations operating in the region.

Since 9/11, the world has seen a marked shift in the global terrorism landscape. Global counterterrorism programs have systematically dismantled the highly centralized terrorist organizations that dominated the security landscape during the late 20th century. In their place, domestic actors, often self-radicalized on the internet, have adapted tactics aimed at evading the massive security structures developed after 9/11. Additionally, the range of Western ideological actors drawing on terrorist tactics have expanded, ranging from Islamist groups to domestic far-right and accelerationist groups. In the last five years alone, the U.S. has experienced such high-profile attacks as the synagogue shootings in Pittsburgh and Poway, a series of lone operator attacks in New York City, the Pulse nightclub shooting, and a vehicle ramming attack at the Unite the Right rally in Charlottesville.

As these examples demonstrate, individual actors have demonstrated their ability to be just as deadly acting alone, without the explicit direction and assistance from established terrorist organizations, and regardless of where they draw their inspiration from. The perpetrators of these attacks were motivated by the propaganda of both white supremacist and Salafi-Jihadi ideologies and drew strategic and tactical guidance from other notable attacks, extremist propaganda and instructional materials, and domestic and international terrorist networks. Teneo Risk assesses that regardless of ideology, these actors pose an ongoing, and persistent, threat to the U.S. homeland.


From Directed, External Operations to Individual, Inspired Attacks

The two decades since the 9/11 attacks has seen a remarkable and evolving terrorist landscape, ranging from operations and plots that were planned and directed by highly centralized groups such as al-Qaeda and its affiliates overseas to lone operator attacks carried out by homegrown violent extremists inspired by al-Qaeda and the Islamic State, and fueled by sophisticated social media recruitment and mobilization campaigns. This evolving terrorist landscape has necessitated a dynamic response to secure potential targets and counter tactical innovations. Teneo Risk assesses that not only will individual actors inspired by extremist groups and ideologies continue to pose the primary threat to the U.S. homeland, but these actors will continue to target both easy-to-access and high-value targets using accessible weapons and tactics.

  • In the decade following 9/11, terrorist plots developed, aided or inspired by al-Qaeda posed a persistent threat to the U.S. homeland. These included the 2005 attacks on the London Underground transport system, the 2004 coordinated train bombings in Madrid, Spain, and the 2001 attempted shoe bombing attack aboard an American Airlines flight from Paris to Miami.
  • Al-Qaeda in the Arabian Peninsula’s Inspire magazine also provided homegrown extremists in the U.S. with practical tips on how to select targets and how to make explosive devices. A feature in the magazine that provided instruction on how to create pressure cooker bombs served as instruction for the perpetrators of the 2017 Manchester, UK Arena bombing, the 2013 Boston Marathon bombing and the 2010 Times Square car bombing attempt.
  • In 2014 the self-declared Islamic State emerged as an offshoot of al-Qaeda in Iraq, following the U.S. withdrawal from Iraq, which in turn ushered in a new era of IS-inspired attacks and plots. In the five years following the founding of the Islamic State’s caliphate in Iraq and Syria in 2014, IS-inspired attacks targeted government and civilian targets across North America and Europe.
  • This era in the post 9/11 terrorism landscape was also marked by the ubiquitous use of social media to spread propaganda, radicalize supporters, and encourage and assist individual actors to carry out attacks.


Threat of White Supremacist and Far-Right Violence

Heightened focus on the upcoming #JusticeForJ6 rally, organized by Look Ahead America, at the U.S. Capitol on September 18th additionally highlights the threat posed by racially and ethnically motivated groups and actors, particularly those ascribing to white supremacist and other far-right ideologies. Despite recently renewed focus on the terrorist threat posed by al-Qaeda and other Salafi-Jihadi actors following the U.S. withdrawal from Afghanistan and the twentieth anniversary of the 9/11 attacks, Teneo Risk assesses that domestic white supremacist and anti-government groups, as well as individuals inspired by these groups, will continue to be the leading terrorist threat to the U.S. homeland in the near term.

  • At the end of 2019, the FBI noted that 2019 was the deadliest year for domestic terrorism since the Oklahoma City truck bombing in 1995. Other agencies within the intelligence and law enforcement community have put out similar assessments, as have think tanks and research organizations. In fact, there is broad agreement that in the last four years, violence linked to white supremacy has eclipsed Salafi-Jihadi violence as the predominant form of terrorism in the United States.
  • White supremacists have been responsible for some of the deadliest mass shootings in the U.S. in recent years, including the 2019 shooting at a Walmart in El Paso, TX; the 2018 shooting at the Tree of Life Synagogue in Pittsburgh, PA; and the 2015 shooting at the Emanuel African Methodist Episcopal Church in Charleston, SC.
  • On January 6, 2021, a range of actors, including individuals affiliated with domestic extremist groups, attacked the U.S. Capitol, destroying property and assaulting law enforcement personnel. Individuals affiliated with the Proud Boys, a white nationalist group, as well as members of the anti-government Oath Keepers and Three Percenters groups were active participants in the assault.


U.S. Withdrawal from Afghanistan May Pose Long Term Risk

On August 30, 2021, the U.S. ended its nearly two decade military engagement in Afghanistan. The official withdrawal, which followed a chaotic week of the Taliban reclaiming territory across the country and taking control of Kabul, has surfaced renewed concerns about al-Qaeda’s resurgence in a Taliban-controlled Afghanistan. While there is no credible or specific open-source intelligence that indicates an immediate heightened risk of terrorist threats to the U.S. homeland from al-Qaeda or the Islamic State’s Khorasan Province, Teneo Risk assesses that there are several threat indicators to monitor and analyze in the short and medium term:

  • The Taliban takeover of Kabul has given Salafi-Jihadi supporters a propaganda win, with al-Qaeda supporters and sympathizers celebrating this victory in online forums and platforms, which in turn could be used as a recruitment and mobilization tool to inspire lone actors.
  • While there is no specific open-source information indicating external operations planning by al-Qaeda or foreign terrorist organizations, official and unofficial statements and propaganda from al-Qaeda and other foreign terrorist organizations in the region can serve as indicators to ascertain intent regarding external operations attacks targeting Western countries.
  • The newly-announced Taliban caretaker administration — led by members of the group’s military leadership and exclusively male, Pashtun, and without representation from other ethnic and religious communities — violates the group’s earlier promise to form an “inclusive" government. The group’s composition raises concerns regarding Taliban willingness to follow through on other promises, particularly regarding denial of permissible conditions for terrorist groups to reconstitute or expand strongholds in the country. For example, Sirajuddin Haqqani —  the leader of the Haqqani Network, a U.S.-designated foreign terrorist organization — is the Taliban government’s new Interior Minister. The Haqqani Network is believed to have coordinated several high-profile attacks and plots in the region and has had close ties to al-Qaeda over several decades.
  • Salafi-Jihadi supporters are not the only extremists who have responded to the Taliban victory in Afghanistan. Right-wing extremists have made comments on social media and deep/dark web platforms regarding the withdrawal, with comments ranging from expressing desire to replicate the Taliban’s victory in the U.S. homeland to expressing concern over potential extremists being able to exit Afghanistan and enter North American and European countries. Right-wing extremists have previously leveraged migration and refugee related developments as tools for recruitment and mobilization and could use the ongoing situation in Afghanistan as a recruitment and mobilization tool.
The views and opinions in these articles are solely of the authors and do not necessarily reflect those of Teneo. They are offered to stimulate thought and discussion and not as legal, financial, accounting, tax or other professional advice or counsel.

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