A dozen airport and Transportation Security Administration (TSA) employees have been arrested for their alleged involvement in a massive cocaine smuggling operation in Puerto Rico, the U.S. Attorney’s Office announced Monday.

The defendants are accused of helping smuggle approximately 20 tons of cocaine through Luis Muñoz Marín International Airport over the course of 18 years, from 1998 to 2016.

The investigation was initiated by the TSA as part of an ongoing effort to target employee misconduct and reduce insider threats.

The operation allegedly involved employees smuggling suitcases through TSA checkpoints at the airport and onto flights, with as many as five mules on some flights and with each mule checking two suitcases in some cases.

Six current and former TSA screening officers have been indicted in the case for their alleged role in smuggling cocaine through X-ray machines and onto airplanes without detection.

Twelve current and former TSA agents and airport workers have been