Washington (CNN) -- U.S. officials warn the threat posed by a lone gunman or a group with small arms is a serious concern to law enforcement.
A joint intelligence bulletin issued by the Department of Homeland Security and the FBI cites the July attack in Norway as one very deadly example. Anders Behring Breivik allegedly shot and killed 69 people on the Norwegian island of Utoya. Authorities say earlier that day a vehicle bomb made by Breivik exploded in Oslo and killed eight people.
The U.S. law enforcement bulletin issued Tuesday warns Breivik's preparations went undetected. The memo, which was obtained by CNN, says Breivik "is believed to have acted alone and used legal methods to procure the vast majority of materials and weapons needed for his operation, successfully avoiding law enforcement suspicion."
In an interview with CNN's Wolf Blitzer on Tuesday, President Barack Obama expressed concerned about lone wolf actors "being able to carry out wide-scale massacres of the sort that we saw in Norway." He called the lone wolf worry "the most likely scenario that we have to guard against right now."
A law enforcement official told CNN the alert sent to state and local law enforcement was a "cautionary kind of thing" and was not the result of intelligence concerning a possible upcoming attack. The official noted the upcoming 10th anniversary of the September 11 terror attacks, but quickly added law enforcement is always worried about catching lone actors before they strike.
The document lays out the singular challenge for law enforcement presented by lone wolves: "Attacks by lone offenders -- which by definition lack co-conspirators, and therefore provide fewer opportunities for detection -- may be more difficult for law enforcement and homeland security authorities to disrupt."
Transportation Security Administration chief John Pistole says the lone wolf scenario remains a concern, and intelligence is the "best tool" to prevent it. "There are a number of trip wires in place around the country that the FBI and others have, state and local police. Those are the best opportunities to detect and deter a possible terrorist. That being said there are those individuals -- lone wolves that I referred to -- that do things on their own," Pistole told CNN.
Speaking before the U.S. Chamber of Commerce at a "See Something, Say Something" public awareness event on Wednesday, Department of Homeland Security Secretary Janet Napolitano said, "We're seeing smaller plots using a variety of techniques ... we're also seeing the rise of activities by individuals who are actually in the country. And they're acting by themselves. And that kind of attack is the most difficult to prevent."
However, the United States has had success detecting lone wolf plots. The bulletin points to the July 27 arrest of an AWOL soldier named Naser Abdo who allegedly wanted to attack soldiers at gathering spots near Fort Hood in Texas. The document says law enforcement found bomb-making materials in Abdo's hotel room but law enforcement was alerted in time to avert an attack thanks to a warning from a gun store employee who found Abdo's behavior suspicious.
But another plot involving Fort Hood was not detected, which resulted in tragic consequences. In November 2009, Army Maj. Nidal Hasan allegedly opened fire in a center where soldiers were preparing to deploy overseas. Armed with two pistols, Hasan allegedly killed 13 people and wounded more than 30 others.
The bulletin also warns terrorist groups overseas "have long favored small unit assault tactics, in which small teams of operatives storm a target using small arms to defeat security." U.S. officials say they have found "no information indicating transnation terrorists have attempted to execute a small-unit assault operation in the homeland."
But the document lists two homegrown plots that were disrupted. One involved six people found guilty of plotting to attack a military base at Fort Dix, New Jersey. A second one listed by DHS and FBI officials concerns two men who allegedly wanted to strike at a Military Entrance Processing Station in Seattle.
CNN's Jim Barnett contributed to this report.