U.S. officials said intelligence gleaned from overseas electronic intercepts and other means points to California as the location of highest concern. U.S. officials expressed fears over the possible use of a range of devices that included biological or chemical weapons and a radiological, or "dirty," bomb, officials said.Department of Homeland Security officials decided the threat was credible enough to order the immediate placement in California of additional outdoor air-sampling sensors that are designed to warn of any release of deadly microbes into the atmosphere. Hundreds of such Biowatch sensors have been in use in a number of California cities and in dozens of others, including Washington and New York, since March.