U.S. Homeland Security Secretary Janet Napolitano is urging Americans to be “vigilant” in reporting any suspicious activity, as investigations continue into what authorities say is a credible but unconfirmed terrorist threat planned to coincide with the 10th anniversary of the September 11, 2001 terrorist attacks.

In a public statement Friday, two days before the anniversary, Napolitano said federal authorities are working work with local and state law enforcement to implement “seen and unseen” measures to mitigate any danger. But she said protecting the U.S. is a “shared responsibility” and all residents can play a role.

Few details have been released about the nature of the threat, but Secretary of State Hillary Clinton indicated al-Qaida is behind it. Delivering a speech on counterterrorism in New York Friday, she described the threat as a report that “al-Qaida again is seeking to harm Americans and in particular to target New York and Washington.”

Authorities in both those cities say they have increased police staffing for the next several days, but said the threat of an attack would not disrupt Sunday's ceremonies for the September 11 anniversary.

Meanwhile, the White House says President Barack Obama does not intend to change his schedule. Mr. Obama plans Sunday to visit the sites of the attacks in New York City, the Pentagon and Shanksville, Pennsylvania.

The White House says President Obama has been continually updated on the threat and that he has called on counterterrorism authorities to redouble their efforts to protect the U.S.

In her speech Friday, Clinton said the country must do more to fight terrorism, but said it cannot and will not live in fear, sacrifice its values or pull back from the world.

Earlier this week, the U.S. government raised the alert level at its domestic military bases ahead of the anniversary.