The warrants were served on two apartments in the 2000 block of Morlan Drive at around 5:30 p.m.
The Napa Special Investigations Bureau had received a tip that a resident there was growing and selling marijuana, and after a three month investigation, agents had enough evidence to obtain a search warrant.
Agents said they found two rooms converted to grow and process marijuana in an upstairs apartment, as well as about a half-pound of processed marijuana, several plants, cash and evidence of illegal sales.
The apartment allegedly belonged to Colby Hamilton, 25, and he was arrested there on suspicion of cultivation and sales of marijuana.
In an apartment downstairs, the agents found several vials of steroids and another half-pound of processed marijuana.
In a closet with some of the marijuana, agents also found an AR-15 assault rifle and loaded magazines, and elsewhere found a military-style ballistic helmet and ballistic vest.
The apartment allegedly belonged to Victoria Rossi, 24, and Ryan Perez, 33. Rossi was arrested on the scene and Perez turned himself into authorities on Wednesday, investigators said.