This comes just after the Pennsylvania attorney general announced a major heroin pipeline bust in York and Blair Counties.
Kilograms of heroin, crack cocaine and cocaine powder were distributed from the mill in Lebanon.
U.S Attorney for the Middle District of Pennsylvania Bruce Brandler said, "I haven't seen one like this before, and I've been in this business for 30 years."
Stolen guns were also found inside the mill.
Brandler said Julio Aviles, Sr., 48, was the owner of the mill.
"He had a number of people who sold the deadly drugs for him. He had packagers, and he had testers as well as brokers and middle men who acted as go-betweens between him and his source of supply in Philadelphia," Brandler said.
Michael Millan-Miranda, 30, of Lebanon, and Israel Nazario, 61, of Philadelphia, were also found guilty of drug trafficking and firearms offenses.
Law enforcement in Lebanon County were given a tip about the operation from an addict.
"It's just not a matter of prosecuting these individuals, that we need to do community outreach, education, prevention measures to stop the demand for these drugs," Brandler said.
Aviles has a prior drug trafficking conviction and could now face a life sentence. Officials said he ran the mill in a routine manner, producing 40-50,000 individual doses of the drug a week.
Brandler said, "With his packagers who acted as factory workers and were paid $500 a week to show up for their shift. They worked in shifts seven days a week packaging heroin."
Daryl Bloom, the chief of the Organized Crime Drug Enforcement Task Force, said, "One of the testers was 8 months pregnant when she was testing drugs. We know that the drugs were mixed with a number of things: morphine, a horse tranquilizer that was mixed, as well as Fentanyl."
The operation was going on for two years until it was shut down in 2015. Brandler said the recent crackdown on major heroin dealers across the country is a priority for President Trump's administration.
"All indications are that we are going to be increasing our efforts in this area," Brandler said.