A Sacramento, California, man was sentenced Thursday to over three years in prison for unlawful manufacture of a firearm and one count of dealing firearms.

Last year, Daniel Crowninshield pleaded guilty to those counts in exchange for federal prosecutors dropping other charges. According to investigators, Crowninshield, known online as "Dr. Death," would sell unfinished AR-15 lower receivers, which customers would then pay for him to transform into fully machined lower receivers using a computer numerically controlled (CNC) mill. (In October 2014, Cody Wilson, of Austin, Texas, who has pioneered 3D-printed guns, began selling a CNC mill called "Ghost Gunner," designed to work specifically on the AR-15 lower.)

"In order to create the pretext that the individual in such a scenario was building his or her own firearm, the skilled machinist would often have the individual press a button or put his or her hands on a piece of machinery so that the individual could claim that the individual, rather than the machinist, made the firearm," the government claimed in its April 14 plea agreement.

Under federal law, it is allowed to manufacture your own firearm (even with a CNC mill), but it is not allowed to do so for others without proper licensing.

In a Thursday statement, Special Agent in Charge Jill A. Snyder, of the Bureau of Alcohol, Tobacco and Firearms, said that Crowninshield "owned and operated a machine shop where he allowed customers with unknown backgrounds to use his machinery to unlawfully manufacture firearms for profit. That activity posed a very dangerous threat to the safety of our communities."