TULSA, Oklahoma - Four men were arrested in connection to a Tulsa drug ring that the FBI described as operating as a pizza delivery business.

Four pounds of suspected heroin were also seized as part of the FBI's two-year investigation into the drug operation, according to a criminal complaint filed Tuesday in the United States District Court for the Northern District of Oklahoma.

The investigation began in October 2015, the filing states. The investigation included the FBI, Drug Enforcement Administration, Tulsa Police Department and the Broken Arrow Police Department.

"This network's distribution model mirrors any nationally recognized pizza delivery chain such as Pizza Hut and Papa John's," according to the filing. "Much like a pizza delivery chain, a (heroin) customer calls a 'dispatcher,' places an order, and receives their product within thirty (30) minutes."

A customer calls a dispatcher and places an order for heroin, the filing reads. The dispatcher accepts the order, then directs the customer a location to meet.

Usually, customers are sent to a public business area with a high volume of vehicular and pedestrian traffic, according to the filing. These locations help the delivery driver to easily blend in.

Common locations included the Wal-Mart Neighborhood Market at 31st Street and Garnett Road, the QuikTrip at 51st Street and Sheridan Road, and the Walgreens at 51st Street and Memorial Drive.

Before the call ends, the customer is provided an estimated time of arrival and — if they request it — information about the vehicle driven by the delivery driver.

After the customer and delivery driver arrive and acknowledge each other, the driver and the customer drive to a nearby residential area and park.

The customer enters the delivery driver's vehicle, and they drive around in the neighborhood, making several turns to ensure they were not being followed by law enforcement, according to the filing.

One-gram balloons costing $100 each were sold in the operation, according to the filing.

Four people were arrested as a result of the undercover operation: Martin Esteban-Flores, 27; Carlos "Tado" Gonzeles Rosales, 27; Guillermo "Tono" Samuel Inda-Perez, 25 and Eder "Sinaloa" Cervantes Garcia, 40.

All four remain in the Tulsa County Jail without bond on ICE holds.

You can view the full filing below: