Department of Homeland Security Secretary Janet Napolitano addresses the US Chamber of Commerce's “Building Resilience through Public-Private Partnership” conference at the chamber's headquarters August 3, 2011 in Washington, DC. – AFP

WASHINGTON: There have been no credible or specific threats against the United States yet ahead of the tenth anniversary of the Sept. 11 attacks, US Homeland Security Secretary Janet Napolitano said on Wednesday.

US officials have been on high alert for possible plots to reprise in some form of the Sept. 11, 2001 attacks in which al Qaeda operatives hijacked commercial airliners and crashed them into the World Trade Center and the Pentagon.

“We have no specific or credible threats involving the 9/11 anniversary to date,” she told reporters during a news conference on an unrelated subject.

Counterterrorism officials have also been worried about retaliatory attacks after US forces killed al Qaeda leader Osama bin Laden during a raid on his compound in Abbottabad, Pakistan in May.

One DHS and FBI bulletin issued to law enforcement agencies after that raid warned that as of February 2010, bin Laden and al Qaeda were discussing the possibility of derailing trains in the United States on the Sept. 11 anniversary this year