The United States raised its alert level at military bases ahead of the 10th anniversary of the 9/11 attacks, as a report warned of "gaps and weaknesses" in the nation's security apparatus.
The Pentagon said it had upped security at installations as "a prudent and precautionary measure," and not in response to a particular threat, as attention began to build towards Sunday's commemorations.
A decade on, America is marking 9/11 with a series of events to honor the nearly 3,000 people who died when hijackers crashed planes into the World Trade Center, the Pentagon, and a field in rural Pennsylvania.
The biggest occasion will be the reading of victims' names at Ground Zero, an event that will be attended by victims' family members, President Barack Obama, former president George W. Bush, and New York Mayor Michael Bloomberg.
Announcing the decision to increase security, a Pentagon official said Al-Qaeda had "focused on holidays and milestone events in the past," noting that the 10-year anniversary "was mentioned in the documents seized" in the US raid that killed Osama bin Laden at his compound in Pakistan on May 2.
The Al-Qaeda leader was reportedly planning a major attack to coincide with the 10-year anniversary of September 11, but US officials have stressed there was no evidence the "nascent plot" ever went beyond the planning phase.
Security is the main concern among organizers of all ceremonies remembering those who died and large crowds are expected as the country reflects on a decade of war abroad and fear of further attacks at home.
Although there have been no attacks on the scale of 9/11 in the United States in the 10 years since, a palpable fear of a similar outrage persists and is often heightened ahead of major events.
An official report on Wednesday said that the Department of Homeland Security created after 9/11 to protect American soil had made progress in the past 10 years but security measures at airports remained inadequate.
"Work remains for DHS to address gaps and weaknesses in its current operational and implementation efforts, and to strengthen the efficiency and effectiveness of those efforts," said the Government Accountability Office.
"DHS does not yet have a plan for deploying checked baggage screening technologies to meet recently enhanced explosive detection requirements," its report said.
The State Department on September 2 issued a worldwide travel alert ahead of the 10th anniversary, calling on US citizens living and traveling abroad to remain vigilant.
Obama's top counter-terrorism advisor John Brennan on Wednesday cautioned that the US must maintain high vigilance on security matters, as "Al-Qaeda is still there," despite bin Laden's death.
US Navy commandos tracked down bin Laden at his hideout in the Pakistani garrison town of Abbottabad and killed him in a raid which many 9/11 victims' families have said gave them a level of closure 10 years on.
A former Saudi Arabia ambassador to the United States, however, said bin Laden's death should have been used as an opportunity to pull American troops out of Afghanistan, where a Taliban insurgency continues to rage.
"It would have been the perfect moment to declare victory and withdraw from Afghanistan ... and not to continue with this endless fight," Prince Turki al-Faisal said at the Center for Strategic and International Studies (CSIS) think tank in Washington.
The 9/11 attacks left 2,977 people dead. The vast majority of those killed, 2,753, were in New York, while 184 people died at the Pentagon and another 40 on Flight 93, which crashed in Shanksville, Pennsylvania.
Those figures exclude the 19 hijackers.