(AP, ABC7) - As the tenth anniversary of the Sept. 11 attacks approaches, federal law enforcement and intelligence agencies are ramping up efforts to make sure history doesn't repeat itself.

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Cited as a top concern is a so-called lone-wolf terrorist, potentially angry over Osama bin Laden's death. Even though there's no specific threat, FBI field offices are reviewing active cases and reaching out to informants.

“The notion of trying to prevent attacks by radicalized Americans, or people in this country lawfully, is almost impossible,” said former 9/11 commissioner James Thompson.

Amtrak is adding heavily armed patrols and bomb detection units at train stations. The TSA is using armed viper teams to patrol airports.

The U.S. has issued a worldwide travel alert that cautions Americans about the continued threat posed by al-Qaida and other groups.The State Department isn't citing any specific threats to the United States that have been identified from al-Qaida or its allies.

But the department says U.S. citizens living or traveling abroad should take precautions because terrorist groups can attack anywhere.

Friday's alert notes that previous attacks were planned for significant dates, presumably to gain maximum public attention.

The U.S. says Americans abroad should register on the State Department's website or at an embassy or consulate. This makes it easier for embassies to reach people in case of emergency.

Another security concern is al-Qaeda in the Arabian peninsula, the group behind the underwear bomb plot and last year's failed attempt to blow up parcel planes.

Football stadiums are reinforcing their security since the first full day of NFL games is Sunday, Sept.11.

Homeland Security Secretary Janet Napolitano released a statement saying the nation remains at a heightened state of vigilance, urging everyone to be on the alert, and to report anything suspicious.