Stephen Tanabe, 47, of Alamo, was arrested at 9:30 p.m. after investigators discovered he might have been keeping and selling drugs, according to a statement from sheriff's spokesman Jimmy Lee.
Tanabe faces charges of conspiracy to possess and sell controlled substances, and possession and transfer of an assault rifle, according to Lee.
His arrest was part of an ongoing investigation into the state Department of Justice Central Contra Costa Narcotics Enforcement Team, or CNET.
"The arrest of Tanabe is a sad day for the office of the sheriff but should not reflect on the integrity or professionalism of the nearly 1,000 other members of the office," Sheriff David Livingston said in a statement.
Tanabe was placed on administrative leave, and he is being held at the Martinez Detention Facility with a bail of $260,000, according to the sheriff's office.
He has been with the sheriff's office for four years.