Security in New York City remained tight Monday. The Department of Homeland Security extended security measures it beefed up last week.
Officials told ABC News that in the last 72 hours, the FBI has questioned and cleared 300 people who fit the profile of men who may have flown into the U.S. to carry out an attack.
Security at the National September 11 Memorial in New York, which opened to the general public Monday, resembles that of an airport.
In Los Angeles, the sight of police officers patrolling the concourse at Los Angeles International Airport was more prevalent.
It was no surprise to seasoned travelers who knew that flying on Sept. 11 would bring increased security.
"My parents didn't really want me flying around this time of year, but I think that's kind of giving in to the fear that drives people, and you can't really give in to that," said Skyler Rousselet of Burbank.
In a statement, LAX officials said they have enhanced the number of uniformed officers at the airport, including those manning checkpoints and conducting random baggage checks. They said they are also taking other security measures that aren't visible to the public.
On Sunday, two commercial flights had to be escorted by military jets after passengers and flight crew reported what they perceived to be suspicious behavior.
On a New York-bound flight out of LAX, passengers that used the bathroom excessively prompted security concerns.
There was a nearly identical incident on a Detroit-bound flight.
Some of the passengers were detained in the incidents, but none were charged.
At downtown Los Angeles' Union Station, security measures over the next couple of days include bomb sniffing dogs and increased staffing.
"Al Qaeda would like to strike on the anniversary or around the anniversary," said Cmmdr. Patrick Jordan of the Los Angeles county Sheriff's Department. "For the transit system, Sunday probably isn't the best time to hit. A weekday would be better when you have a lot of passengers."