Washington ? As the nation prepares for the 10-year anniversary of the 9/11 attacks — a date al-Qaida has cited as a potential opportunity to strike again — security is intensifying at airports, train stations, nuclear plants and major sporting arenas around the country.

“At this point there is no specific credible threat, but that doesn’t mean we are relaxing at all in terms of our vigilance,” said John Brennan, President Barack Obama’s homeland security and counterterrorism adviser.

“We are concerned about the lone actors that are out there, we are concerned that al-Qaida or others may try to take advantage of the 9/11 anniversary events,” Brennan told The Associated Press on Wednesday.

“We’re looking at all different angles — what might have been planned for a while, we’re still looking for indications that there might be something out there, but we are very interested in seeing whether or not there’s any indication whatsoever of a lone actor and that’s much more difficult to pick up.”

The security ramp-up around the country underscores a shift in policing focus since the attacks a decade ago. Officers and emergency responders have been trained in detecting suspicious activity that could uncover a terror plot, aware that the threat has changed in part from an organized large-scale attack using airliners as missiles to the potential for smaller, less sophisticated operations carried out by affiliated groups or individuals.

Metropolitan areas are on alert.

“Throughout the city, whether it’s the ports or the airports or venues or whatever, you will see an increase in awareness, an increase in resources at strategic places,” said Mark Eisenman, assistant chief over the homeland security command for the Police Department in Houston, home to the country’s largest port. “We are certainly aware of the threats and the concerns, and we’re much more willing to share information than probably ever in the past.”

Some of the first information gleaned from Osama bin Laden’s compound after he was killed in May indicated that, as recently as February 2010, al-Qaida considered plans to attack the U.S. on the 10th anniversary of the 9/11 airliner strikes. But counterterrorism officials say they believe that planning never got beyond the initial phase and they have no recent intelligence pointing to an active plot.

On Wednesday, vendors at Los Angeles’ regional transit hub, Union Station, were being briefed by law enforcement on ways to be aware of suspicious activities over the next few weeks, said Commander Pat Jordan, chief of the transit services bureau at the L.A. County Sheriff’s Department.

There will be increased law enforcement presence on L.A. transit systems during the “threat window,” with bomb sniffing dogs, and random baggage searches, he said, adding, “You can’t be complacent.”

Transit employees in L.A., like riders around the country, are told that if they see something, they should say something. And three weeks ago, the department held an exercise with an active shooter scenario similar to the tactics terrorists used in the deadly 2008 attacks in Mumbai. In the transit environment, Jordan said, some of the greatest threats could come from gunmen and the use of explosives hidden in backpacks.