The U.S. Department of Homeland Security, in a special terror bulletin, warned of "insider" threats to critical infrastructure, such as electrical grids and chemical plants.
In the warning issued in late July it said extremists had in the past obtained insider positions in the utility sector and that outsiders had also tried to "solicit" utility employees to commit damage.
The U.S. State Department has issued a worldwide caution.
"Extremists have targeted and attempted attacks on subway and rail systems, aviation and maritime services," it said. "In the past several years, these types of attacks have occurred in cities such as Moscow, London, Madrid, Glasgow and New York City."
In addition to extra federal government measures being put in place as the anniversary of al-Qaida's massacres, local authorities are also acting. In California, for example, San Diego County has launched a public awareness campaign -- Eight Signs of Terrorism -- that identifies possible signs of terrorist activity and urges people to report suspicious activities to authorities.
"Common sense dictates … that we should all be alert and be aware of our surroundings," County Sheriff Bill Gore told KPBS radio in San Diego. "This is kind of a … neighborhood watch program, if you will."
Fears of a terrorist incident aren't unfounded. Osama bin Laden, the founder of al-Qaida, was killed in May but the organization survives, as does affiliates groups and jihadists around the world, including within the United States.
"The key shift in the past couple of years is the increasingly prominent role in planning and operations that U.S. citizens and residents have played in the leadership of al-Qaida and aligned groups and the higher number of Americans attaching themselves to these groups," Bruce Hoffman, a terrorism expert at Georgetown University wrote in a report for the Bipartisan Policy Center in Washington.
Al-Qaida and its allies "have been able to establish at least an embryonic terrorist recruitment, radicalization and operational structure in the United States with effects both at home and abroad," Hoffman said.
Anwar al-Awlaki, the head of al-Qaida in the Arabian Peninsula, was born and raised in the United States and actively recruits and encourages Americans for jihad over the Internet. Virginia-born U.S. Army psychiatrist Maj. Nidal Malik Hasan, who is accused of killing 13 people in a shooting spree at Fort Hood, Texas, in 2009 is said to have had e-mail exchanges with Awlaki.
Carlos Bledsoe, aka Abdulhakim Majid Muhammad, who spent time in Yemen and claimed to be a member of AQAP, admitted to killing one Marine and wounded another in Little Rock, Ark., during a "lone wolf" attack on a military recruiting station.
Indeed, most lone wolf attacks had at one time briefly left the United States for the Middle East or South Asia for rudimentary terrorist training, including convicted Times Square bomber Faisal Shahzad, who quit his financial analyst job in Connecticut to do so.
Terrorism experts say policies, procedures and actions by the United States have degraded the ability of terrorist groups to mount a large-scale attack reminiscent of the attack on New York's World Trade Center and the Pentagon but they still can try mass casualty operations using the so-called lone wolves.
One attack that could be attempted by a terrorist cell or lone wolves is the derailing of U.S. trains. Documents seized by bin Laden's Pakistan hideout by U.S. troops indicated plans were afoot to do so to coincide with the 9-11 anniversary.
Interestingly, the Times Square bomber and Nigerian Umar Farouq Abdulmutallab, alleged to have tried to blow up a Detroit-bound airliner, were both caught as a result of information from, or intervention by, ordinary people.
So, too, Naser Jason Abdo, the AWOL U.S. Army private arrested last month while allegedly planning to bomb a restaurant frequented by Fort Hood soldiers. The tipster: an employee of a local gun store who became suspicious of Abdo as he was buying black powder and other materials that could be used in the making of a bomb.