Scientists have been secretly testing radiation levels in major US cities as part of the latest security alert, the Washington Post has reported.
Previous scares over anthrax led to sights like this
The newspaper says officials feared a radioactive "dirty bomb" could target New Year celebrations.
It says the government sent out dozens of nuclear scientists with detection equipment hidden in briefcases and golf bags to check for radioactive material.
The US has raised the security alert to "orange", the second-highest level.
US Homeland Security Secretary Tom Ridge raised the alert level on 21 December warning of a "high risk of terrorist attacks".
The following day, scientists were sent out to US cities, the Washington Post reported, covering Washington, New York, Las Vegas, Los Angeles and Baltimore.
The first and only alert came in Las Vegas on 29 December, when detection devices picked up a trace of radiation, the newspaper says.
The White House was notified, but the radiation was found to have come from a cigar-sized radium pellet, used to treat cancer, that a homeless man had found and hidden among his belongings.
The latest security alert is believed to have been due to intelligence "chatter" monitored by the Department of Homeland Security.
New Year fears
According to the Washington Post, officials admit that one of the key challenges they face is trying to determine whether al-Qaeda is planting false clues as a diversion or to test the response of the authorities.
New Year's Eve saw unprecedented levels of security in the US, with flight restrictions ordered over several cities.
Security fears have also led the cancellation of several flights into the US from Britain, France and Mexico.
A dirty bomb uses a conventional explosive device to spew out radioactive material over a small area. Officials in the US and Britain believe al-Qaeda has been working for some time to gain the ability to explode a radioactive bomb.
Research by BBC Two's Horizon programme last year looked into how a dirty bomb attack might affect London.
The programme showed such an attack would wreak panic in built-up areas, see large areas contaminated and closed off and result in long-term illnesses such as cancer.