Two suspected leaders of a multi-state drug trafficking ring have been charged for their role in the operation, as investigators said the pair sent hundreds of pounds of crystal meth and marijuana through the U.S. Postal Service.

Brian Holt, 42, and Lorraine Zeno, 36, both of Ambler, Pennsylvania, were arrested Nov. 8 and charged with intent to deliver and corrupt organizations, among other charges, the Montgomery County District Attorney’s Office announced Wednesday.

Three others — Leonardo Fernandez, 34; Joel Hills-Garcia, 29; and Patrick Fry, 43, all of California — also were arrested in the bust. Investigators have dubbed the group “The Big Five." Those arrested in California will be extradited to Montgomery County to face charges, officials said.

Fifteen others involved in the operation either were arrested or are being sought by law enforcement.

Investigators have said that since at least April 2016, the group has sent more than 350 pounds of crystal meth and marijuana through the U.S. Postal Service.

An investigation into the shipments started when the Postal Inspection Service discovered a pound of meth shipped in an Express Mail package that was headed from California to Montgomery County.

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Investigators used court-authorized wiretaps on Holt's, Zeno's and Fernandez's cell phones, which allowed officials to discover some of their customers and learn more about the operation.

Police said the suspects in California would produce the drugs, then send them to Holt and Zeno to sell in Pennsylvania’s Montgomery, Bucks and Philadelphia counties.

“Bottom line: crystal meth, marijuana, and the mail don't mix,” Bucks County District Attorney Matt Weintraub said.

District Attorney Kevin Steele said in a news release that “Meth is making a resurgence. ... Drug traffickers make the false assumption that using the mail or hiding drugs in car batteries will keep them safe from detection. They are wrong. And we are on to them.”

Preliminary hearings set for Holt and Zeno are set for Jan. 18 and Jan. 29, respectively. Both suspects failed to post bond and are being held at county correctional facilities in Pennsylvania.

Eight overdose deaths in Montgomery County attributed to crystal meth happened in 2016, according to the district attorney's office.