The American Civil Liberties Union, known as a watchdog for the separation of church and state, wants to make sure that prisoners have access to religious material.

In a letter sent last week to the U.S. Department of Justice’s Office of Information and Privacy, the ACLU demanded that the federal Bureau of Prisons release records related to alleged attempts by prison officials to purge religious material from prison chapel libraries.

The demand follows what the ACLU says was an inadequate response by prison officials to a Freedom of Information Act request by a California graduate student writing a thesis on the censorship of religious materials in federal prisons.

According to the ACLU Joshua C. Harris, a master’s degree candidate in religion at Claremont Graduate University, is writing a thesis on the 2007 implementation of the Standardized Chapel Library Project, which authorized BOP officials to purge from prison chapel libraries any material that was not on a list of “acceptable” publications that the libraries could maintain. Among those titles banned at the time, the ACLU says, was Maimonides’ “Code of Jewish Law.”

“The refusal of prison officials to provide a full accounting of their rationale for banning religious material is just the latest example of an ongoing effort to secretly and unconstitutionally censor material they consider to be unacceptable,” David Shapiro, a staff attorney with the ACLU National Prison Project, said in a statement. “To deny prisoners their constitutional right to access religious materials is bad enough. But to attempt to do so in a way that skirts transparency and prevents the public from knowing what they are doing is entirely unacceptable.”

Harris filed a FOIA request in April asking for “any/all documents that detail the reasoning behind, and implementation of” the Standardized Chapel Library Project, according to the ACLU. The prison bureau gave him four documents.