WASHINGTON (AP) — 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.

“Our role at the local level has become much more important over the past year, as we realized the potential for the insider, domestic threat — somebody who is already here in the United States,” said Washington Metropolitan Police Department Chief Cathy Lanier. These insiders, already living inside the country, would have a shorter planning cycle than a terrorist organization plotting to attack the U.S. from overseas, she said, and therefore local law enforcement officers are critical in catching a plot in their community before it’s too late.

In Phoenix, police will be doing more patrols around the region’s nuclear power plant, airports and other critical sites that, if attacked, could affect the city, said Bill Wickers, sergeant at the homeland defense bureau of the Phoenix Police Department. Messages on the department’s internal television station include reminders of what constitutes suspicious activity, such as someone making drawing a diagram of a piece of important infrastructure or someone wearing a heavy coat while it’s 115 degrees outside.

“The heat’s been turned up,” Wickers said.

And there’s a rapid response team of hazardous material technicians, special weapons and tactics and bomb operators ready to go, Wickers said, adding that the city used federal money that became available after 9/11 to help fund that team and other counterterrorism measures.

In Florida, there will be extended staffing at the state’s fusion center — an intelligence-sharing operation established after 9/11 that places law enforcement from around the state, as well as representatives from the FBI, Homeland Security Department and Transportation Security Administration in one building, said the center’s director, Robert LeFiles.

The police chief in Arlington, Va., home to the Pentagon, which was attacked on 9/11, called the weeks surrounding the anniversary a time of heightened awareness.

“Ten years ago changed the world for us, and we should all be consciously aware of what’s going on around us,” said Douglas Scott.

New Yorkers will see more police officers on patrol in and around ground zero, where the World Trade Center towers stood, said Police Department spokesman Paul Browne. The department also plans an increased show of force in the subways, always considered a potential terror target.

Plans to tamper with an unspecified U.S. rail track so that a train would fall off in a valley or from bridge were found on handwritten notes pulled from bin Laden’s Pakistani hideout in May. The al-Qaida planners noted that if they attacked a train by tilting it, the plan would succeed only once because the tilting would be spotted the next time. U.S. counterterrorism officials believe these ideas never got off the drawing board.

“It’s been a long buildup as we approach the anniversary of 9/11,” said Sean Duggan, assistant chief at the Scottsdale, Ariz., Police Department. Duggan said his department gets daily updates from the FBI and Homeland Security Department. But over the past two months, the focus has been on the 10th anniversary of the terror hijackings, as events are planned around the country to commemorate the nearly 3,000 people killed in the 2001 attacks. “We know this is a significant date,” he said.

Associated Press writer Tom Hays in New York and Erica Werner in Washington contributed to this report.