The baggage handlers are among 14 people charged with conspiracy to distribute marijuana, according to a criminal complaint.
"The co-conspirators were described as a drug trafficking organization determined to use the special access some of them had been granted as baggage handlers at the Oakland International Airport to circumvent the security measures in place at the airport," according to a Monday news release from the U.S. Attorney's Office. "As alleged in the complaint, the baggage handlers entered the Air Operations Area of the Oakland Airport while in possession of baggage containing marijuana."
The case is the latest to spotlight how employees at airports are screened.
Like at most airports, the baggage handlers in Oakland do not pass through a security-screening checkpoint, which allowed them to use their airport badges to smuggle in the drugs and then give them to passengers who were part of the scheme, which began in July 2012, the complaint said.
The passengers allegedly took the drugs in their carry-on luggage on Southwest flights.
During the investigation, authorities seized marijuana from suspects who landed at airports in Nashville, New Orleans, Phoenix and Little Rock, Arkansas.
The drugs were sold in various cities and then the proceeds were deposited in bank accounts, according to the complaint.
In a statement, Southwest Airlines said that it "does not tolerate any behavior or activity that could jeopardize the safety and security of our operation. We are fully cooperating with authorities and will continue to work with law enforcement, airport authorities, and our security partners to perform due diligence in upholding high security standards."
Oakland International Airport, in a statement, said it has been cooperating with authorities and "we're pleased to see the investigation concluding and the people involved held accountable."
The airport said that it has a "robust, multi-layered security program that exceeds federal requirements and uses a variety of measures to protect the safety and security of its employees, customers, facilities and operations."
An airport spokesman told CNN that it alerted authorities to the scheme, but would not discuss any specifics of the case.
A CNN investigation earlier this year revealed that only two major airports in the United States, Miami and Orlando, conduct full employee screening by requiring employees to pass through a metal detector.
The issue of employee screening was raised after a Delta Air Lines baggage handler at Hartsfield-Jackson Atlanta International Airport and a passenger were arrested last December for allegedly smuggling guns onto planes headed to New York.
In response, the airport began checking employee bags, reduced the number of doors that access the ramp and increased random screening of employees.
A Transportation Security Administration committee also conducted a nationwide review of airports and concluded that full employee screening was not cost-effective. Instead, it recommended stepped-up random screening of employees and more frequent criminal background checks for anyone with an airport security badge, among other security measures.