In the wake of the Boston marathon bombings much attention has focused on domestic and foreign intelligence gathering, analysis, and sharing as well as law enforcement responses to terrorism. Meriting further focus, though, is the role of the public in informing authorities regarding prospective terrorist threats. Among pre-terror incident indicators the public should report to authorities include: terrorists conducting surveillance, gathering information, testing security, acquiring supplies and funds, acting suspiciously, undertaking dry runs, and getting into position to undertake an attack.

Since 9/11 the public has often recognized these signs of terrorism, informed law enforcement, and aided in preventing potentially large-scale incidents. The case studies shared here are illustrative of the benefits of the public forewarning law enforcement involving alleged suspicious activities, including actions that appear incongruous with standard practices.

A terror cell planned to undertake attacks at Ft. Dix, New Jersey, and other locations in the U.S. Employees at Circuit City approached law enforcement over concern that a customer sought to convert video of the operatives training with weapons and taking about violent jihad from one medium to another (VHS to DVD). Following the use of an informant and undercover agent a sting was forged. Convicted of conspiracy to kill military personnel and weapons charges, four of the five cell members were sentenced to life in prison.

 A chemical supplier called the FBI after Khalid Aldawsari, a Saudi Arabian citizen and a former student at South Plains College in Texas, tried to purchase the toxic chemical phenol, an ingredient in making the explosive trinitrophenol, TNP. A freight company called the police after Aldawsari requested that the firm allow him to pick up the phenol order at their office. Electronic surveillance exposed that he used various email accounts to research explosives and targets. Ultimately, Aldawsari was convicted of attempted use of a weapon of mass destruction and sentenced to life in prison.

 A one-time suspected 20^thhijacker on 9/11, Zacarias Moussaoui, pled guilty to various terrorism charges, including conspiracy with al Qaeda to hijack and crash planes into buildings in the U.S. He received a sentence of life in prison. Authorities initially gained custody of Moussaoui following a call from a Minnesota-based flight school that Moussaoui told instructors he was interested in steering a plane but not learning how to takeoff or land.

 Mohamed Alessa and Carlos Almonte, who pled guilty to conspiracy to commit murder outside the U.S. on behalf of a foreign terror organization, al Shabaab, came to the attention of authorities following a tip from the public. An undercover agent met with the two and recorded their plans to travel to Somalia and join the terror group. Alessa and Almonte were sentenced to prison terms of 22 and 20 years, respectively.

Shahawar Siraj and James Elshafay were convicted of conspiring to bomb the Herald Square subway station in New York City, and sentenced to 20 and 5 years in prison, respectively. New York City Police Department received a tip about Siraj, who worked at an Islamic bookstore in Bay Ridge, New York. Siraj was reported to have articulated anti-U.S. tirades. Following an investigation of Siraj, he and Elshafay were introduced to a confidential informant, who facilitated a terror sting operation.

Amin el Khalifa was sentenced to 30 years in prison for attempting to undertake a suicide bombing at the U.S. Capitol. El Khalifa’s landlord called law enforcement in relation to threats by the tenant against him and suspicion that el Khalifa was making bombs in his apartment. Following interactions with an informant and undercover agent, el Khalifa proposed several plots before choosing to target Washington, D.C.

 As illustrated, reporting upon possible telltale signs of terrorist activity serves society interests. The investigation of the Tsarnaev brothers in the Boston attacks has brought this truism to full view, as a month before the incident Dzhokhar allegedly told one of his friends that he knew how to make bombs. Even more troubling, Dzhokhar’s three friends, upon learning of the Tsarnaev’s role in the incidents, disposed of items, including a laptop, which would incriminate the younger Tsarnaev.

The responsibility for protecting society against acts of terrorism does not rest solely with government. It is everyone's responsibility. So, if you see something, say something.

 Dean Alexander is director of the homeland security research program, and Terry Mors is director and professor, at the School of Law Enforcement and Justice Administration, Western Illinois University.