Marquez, who has previously denied his connections with Farook, admitted to making a false statement as part of his straw purchases of the high-powered rifles, which were eventually used in the deadly shooting in San Bernardino.
"With this plea, Enrique Marquez Jr. will be held accountable for his role in plotting terrorist attacks on American soil with Sayed Rizwan Farook in 2011 and 2012, attacks which were, fortunately, not carried out," the release stated on Tuesday.
Farook and his wife Tashfeen Malik attacked San Bernardino County Department of Public Health training event and holiday party, of about 80 employees, in a rented banquet room. 14 people were killed and 22 were seriously injured.
According to the release, Marquez remained in jail since he was ordered detained at his initial court appearance in the case in December 2015, and now faces a statutory maximum sentence of 25 years in US federal prison.