The nation's top law enforcement officials warned today that al Qaeda may have plotters already inside the United States.
"We have to assume that there are persons out there that want to attack us," said FBI director Robert Mueller.
Homeland Security Secretary Michael Chertoff said police chiefs have been told to review all the intelligence the federal government has given them in the last two years about al Qaeda tactics.
Chertoff told ABC News: "We've seen them attack in London, for example. We've seen them attack in Spain. We've seen them attack elsewhere, so I think we have to operate on the assumption that they do have some capability and they certainly have the intent."
The police chiefs in the nation's two largest cities say they are most worried about commuter trains and airports.
"Close to 50 percent of terrorist attacks in the last 15 years have been directed at transportation facilities," New York Police Commissioner Ray Kelly told ABC News in an exclusive interview. "So, we have to and we are paying additional attention to our transportation system."
Referring to LAX, Los Angeles Police Commissioner William Bratton said, "This airport remains probably the most significant target within the city of Los Angeles."
But police officials say there are not ramping up security across the board because there is no specific intelligence about an al Qaeda plot.
"I think you have to react to specific information, to specifics of a threat," said Kelly. "There's no specific information in this message."
Chertoff insisted that the country has made security upgrades since the attacks of 9/11. He admitted though that a number of vulnerabilities remain.
"Things we're focused on now very seriously are chemical plants -- we're working with Congress on legislation there -- rail security, something which I think we want to make sure we make some progress on in the near future," he said.
Law enforcement remains concerned even though there is little evidence that bin Laden still runs al Qaeda.
Today another audiotape was released from bin Laden's No. 2 in command, Ayman al Zawahri. But the tape does not appear to be recent, and some question its timing -- one week after he was targeted by missile strikes in Pakistan.
"This is a sign of desperation and suggests that perhaps Zawahri was injured or killed, or at the very least on the run," said terrorism analyst and ABC News consultant Alexis Debat.
The FBI is now reviewing all its leads, trying to make sure no clues have been missed.
ABC News' Pierre Thomas filed this report for "World News Tonight."