The following is from a 7 February 2012 report by the U.S. Government Accountability Office.
Container Security Programs Have Matured, but Uncertainty Persists over the Future of 100 Percent Scanning
What GAO Found
As part of its efforts to identify high-risk cargo for inspection, CBP uses various sources of information to screen containers in advance of their arrival in the United States. For example, in 2009, CBP implemented the Importer Security Filing and Additional Carrier Requirements to collect additional information for targeting. The additional cargo information required, such as country of origin, is to be provided to CBP in advance of arrival of the cargo containers at U.S. ports. In September 2010, GAO recommended that CBP establish milestones and time frames for updating its targeting criteria to include additional information. In response, CBP updated its targeting criteria in January 2011.
This report is available in its entirety at: http://www.gao.gov/products/GAO-12-422T