A previous letter[1] supposedly from the same person stated that he was under surveillance before arrest, and thus "black-bag cryptanalysis"[2] is quite probable, particularly due to SDAT being a well resourced domestic spy agency.

"the SDAT explains that they began to tail me and the other comrade (quickly exonerated) starting last January"

[1] https://anarchistnews.org/content/update-letter-ivan-alocco

[2] https://en.wikipedia.org/wiki/Black-bag_cryptanalysis