U.S. officials on Thursday said they were investigating a credible but unconfirmed threat that Al Qaeda was planning to use a car bomb to target bridges or tunnels in New York City or Washington to coincide with the 10th anniversary of the Sept. 11 attacks, the first tip of an "active plot" around that date. The Homeland Security Department said the threat is credible and specific but unconfirmed. The nation's terror alert level has not changed, but raising it was under consideration Thursday night. Law enforcement officials were investigating three people who recently entered the U.S. The threat was received by the U.S. intelligence community late Wednesday night, officials said.