In response to the anthrax mailings of 2001, SRI quickly developed a prototype of a mail fingerprinting system. This system is based on SRI-developed algorithms that identify mail pieces with an address handwriting style similar to that of a known positive mail piece, using a semi-automatically generated decision tree. The degree of similarity is calculated from a set of specially designed address-block, line, and character features. SRI also developed novel algorithms for the robust computation of these features, even when the handwriting style is slanted, uneven, or inconsistent. When trained on one of the anthrax mail pieces and tested on 85,000 mail piece images, the software successfully identified the three known anthrax-carrying mail pieces and produced only three false positives. Figure 1 shows the detection process.