Homeland Security Secretary Janet Napolitano has put out a warning that the U.S. is at a heightened state of vigilance, leading up to the 10th anniversary of the 9/11 attacks.

As we approach the tenth anniversary of the September 11, 2001 terrorist attacks, the safety and security of the American public remains our highest priority, Napolitano said in a statement released Friday through the Department of Homeland Security. While threats remain, our nation is stronger than it was on 9/11, more prepared to confront evolving threats, and more resilient than ever before.

While there is no specific or credible intelligence that al Qaeda or its affiliates are plotting attacks against the United State to coincide with the 10-year anniversary of 9/11, we remain at a heightened state of vigilance and security measures are in place to detect and prevent plots against the United States should they emerge, the statement continued.

In July, it was reported that documents discovered by Navy SEALs in Osama bin Laden's Abbottabad hideout showed that he, along with top deputy Attiyah Abd al-Rahman were plotting an additional Sept. 11 attack.  The documents revealed that bin Laden was aiming for major U.S. cities such as Los Angeles, and was considering using either planes or trains.  

However, military officials have noted that bin Laden's clout in al-Qaida was waining, pointing out that he was more of a figurehead than a commander by the time of his death.

Earlier this week, Amtrak announced  they were stepping up their security measures during the Sept. 11 anniversary period by conducting more passenger and baggage screenings, increasing the use of K-9 explosive detection teams, and expanding police patrols at stations, on trains, along the tracks and other key rail infrastructure.

Despite the warning, the Secretary of Homeland Security has not issued an elevated or imminent alert on the National Terrorism Advisory System.  No report from the FAA has said security measures will be heightened in airports.