The government has repeatedly refused to identify the witnesses in any public papers, saying that if it does, the Sinaloa drug cartel, which Mr. Guzmán ran for 20 years, could easily seek revenge.

Late last month, for example, when the government provided basic information on the witnesses to Mr. Guzmán’s lawyers, it did so in a 100-page memo, almost half of which was blacked out by redactions. Secrecy has so suffused the case that when the defense responded to the memo three days later, asking to learn more about the witnesses, the document was filed under seal.

The chief complaint by Mr. Guzmán’s lawyer, A. Eduardo Balarezo, is that the government is planning to withhold the witnesses’ identities until the eve of trial. He has argued that doing so will hinder his ability to investigate their claims and devise a defense against them.