U.S. counterterrorism officials are investigating what they call a credible but unconfirmed terror threat planned to coincide with the 10th anniversary of the September 11 terrorist attacks on the United States.

Authorities said Thursday they had discovered information suggesting the “credible threat” may focus on Washington or New York. Officials say they suspect three individuals, including a U.S. citizen, entered the United States in August intending to attack with a car bomb.

Late Thursday, New York Mayor Michael Bloomberg said the city has heightened security in response to the threat. But despite the potential danger, he encouraged New Yorkers to continue their lives as usual.

The head of the Federal Bureau of Investigation's New York Division Jan Fedarcyk also refused to name any specific group behind the threat. But she said information gathered in the U.S. raid on Osama bin Laden's compound in Pakistan last May showed that groups such as al-Qaida have an interest in specific dates.

The White House said President Barack Obama has been continually updated on the threat. A White House official told reporters the government already had “significantly enhanced” security and readiness efforts in advance of the September 11 anniversary, but the President Thursday called on counterterrorism authorities to redouble their efforts to protect the U.S. from any attacks.

Officials say the threat is unconfirmed and that they are continuing to investigate. They say they are considering whether to brief local police about what to look for and whether to make the information public. The government is also considering whether to raise the country's terrorist alert level.

Earlier Thursday, Homeland Security Secretary Janet Napolitano said intelligence officials have lately picked up more “chatter” on terrorist websites. She said the U.S. is taking all of the talk seriously.

Earlier this week, the United States raised the alert level at its domestic military bases as a precaution.