Federal authorities on Tuesday announced the arrests in the case of Laredo businessman Robert Jacaman Sr. and his wife, Veronica.
An 11-count indictment returned last month names the couple, Atanacio "None" Gonzalez-Torres, Jose Luis Estrada, Carlos Picazio and Mario Rodriguez.
Gonzalez-Torres was arrested Sunday. Estrada, Picazio and Rodriguez surrendered last week. Estrada and Picazio were ordered released on $100,000 bonds.
The Jacamans are charged with conspiring to provide and providing ammunition and firearm magazines, selling ammunition to illegal immigrants and to a convicted felon and selling an assault rifle to a convicted felon.
The four co-defendants are also charged with conspiracy. Officials did not immediately provide information on attorneys for them.