Enrique Marquez has already pleaded to conspiracy and lying about two rifles he bought that were later used in the 2015 San Bernardino deadly mass shooting.
Federal officials are asking the court to sentence him to 25 years.
Marquez, 26, may not have had knowledge of the specific terrorist attack, but in a court document he admits "could have done something about it."
Marquez bought the assault rifles used that day when 14 people were killed and 22 others were wounded.
The gun purchase was made more than four years prior to the attack, when Marquez and one of the eventual shooters, Syed Farook, were planning other terror schemes. That included targeting a cafeteria at Riverside City College and planning to attack people stuck in rush hour traffic on the 91 Freeway.
But when four other Inland Empire men were arrested for terrorism-related charges, Marquez was scared off.
Nevertheless, he's the one who bought the weapons used that day. Prosecutors say that makes a 25-year prison sentence appropriate.
Relatives of victims are weighing in on the sentence in court documents. Some believe even 25 years isn't enough, as he could be free by age 51.
The father of one victim wrote the judge: "I implore you to consider sentencing this monster to a facility befitting the severity of the crime. ... Please do not send him to anything short of an ugly cell somewhere distant from anything that might provide him any comfort."
Sentencing is scheduled for April 30.
Victims' family seeking stiff sentence for man who helped San Bernardino gunman