In Thornburgh v. Abbott (1989) 490 U.S. 401, federal regulations allowed prison officials to block prisoners from receiving publications that were detrimental to institutional security. (Id. at pp. 404-405.) This specifically included publications that described how to build weapons, how to escape from a prison, or how to manufacture drugs. (Id. at p. 405, fn. 5.) Conversely, the regulations specifically forbade blocking publications solely because their content was religious, philosophical, political, social, sexual, unpopular, or repugnant. (Id. at p. 405.) The Supreme Court held that these regulations were content neutral. It recognized that the determination of whether a publication was permitted or prohibited "turned, to some extent, on content. But . . . 'neutrality' . . . is intended to go no further than . . . that 'the regulation or practice in question must further an important or substantial governmental interest unrelated to the suppression of expression.' Where, as here, prison administrators draw distinctions between publications solely on the basis of their potential implications for prison security, the regulations are 'neutral' in this technical sense . . . ." (Thornburgh v. Abbott, supra, 490 U.S. at pp. 415-416, )