LOS ANGELES—A former U.S. Marine from Whittier who prosecutors say led a six-man scheme to smuggle high-powered weapons and ammunition to one of the most feared transnational drug cartels in Mexico was sentenced Jan. 8 to eight months behind bars.

Marco Antonio Santillan, 53, pleaded guilty in May 2022 to conspiracy to violate export regulations and conspiracy to commit money laundering, according to the U.S. Attorney’s Office.

The defendant’s arrest in January 2022 was the result of a federal criminal investigation dubbed Operation Semper Infidelis, a play on the Marines’ famous motto.

In conjunction with the arrest, Los Angeles authorities unsealed a 23-count federal indictment charging six defendants with conspiring to violate federal export laws by illegally bringing weapons and ammunition to cartel operatives in Mexico.

The indictment said Mr. Santillan ran the gun trafficking operation that used drug money to purchase assault rifles, hundreds of thousands of rounds of assault rifle ammunition, and machine gun parts and accessories. Some of the items were smuggled into Mexico.

The defendants smuggled “sophisticated weaponry out of the United States to one of the most violent cartels in Mexico whose members target not only rival gangs, but innocent Mexican citizens and Mexican law enforcement,” federal officials said.

Among the five other defendants is Marco Santillan Jr., 31, of Pahrump, Nevada, who is the son of the ring’s leader. He pleaded guilty to federal charges in downtown Los Angeles two years ago and was sentenced to 57 months behind bars.

During the Semper Infidelis investigation, authorities seized six assault rifles, more than 250,000 rounds of assault rifle ammunition, and over $300,000 worth of weapons parts and kits to assemble several six-barrel rotary machine guns capable of firing up to 6,000 rounds per minute, according to the U.S. Attorney’s Office.