Bomb threats brought transportation to a halt on Tuesday.

Amtrak said three of its trains received threats -- The Palmetto in North Carolina, and Silver Meteors in Florida and Philadelphia. All three were searched, but nothing was found. Each was allowed to resume after searches.

Northwest planes in Los Angeles, Detroit, Miami and Memphis were all subject to search, following similar threats.

A Transportation Security Administration spokeswoman said the threats were not credible, but the searches were done as a precaution.