(CNN) — How were inmates inside a Texas prison allegedly able to make cell phone calls, plot crimes and acquire drugs?
With the help of 17 former corrections officers who once worked at the prison, according to a federal grand jury indictment unsealed this week.
Now the former officers and 12 others have been arrested and charged with racketeering after a four-year investigation authorities dubbed Operation Prison Cell.
The former guards are accused of involvement in a smuggling scheme that undermined the justice system, U.S. Attorney Kenneth Magidson told reporters Wednesday.
"Prison is a place where inmates are supposed to do their time," he said, "not where they do their crime."
Using the cell phones guards smuggled in, prison inmates orchestrated and facilitated killings, home invasions and drug trafficking, said Angela Dodge, a spokeswoman for the U.S. Attorney's Office.
The indictment released Wednesday accuses 13 former corrections officers of smuggling cell phones into the McConnell Unit Prison in Beeville, Texas, about 200 miles southwest of Houston.
"The employees supplied the phones, knowing the inmates would use the phones to conduct illegal activities," the indictment says.
A four-year investigation into the "culture of corruption" at the prison began after authorities caught gang members trying to transport stolen cars that were destined for Mexican cartel members across the border, the U.S. Attorney's Office for the Southern District of Texas said Wednesday.
Prison inmates using illegal cell phones were behind the thwarted attempt to move the stolen cars into Mexico, prosecutors said.
Nationwide, the smuggling of contraband cell phones into prisons is a growing problem, the U.S. General Accountability Office said in a 2011 report, noting that the number of smuggled phones seized had quadrupled in a three-year period.
In 2007 an inmate at a Maryland detention center ordered the murder of a witness to his crimes using a contraband phone, the report said.
And in 2008, a death row inmate in a Texas state prison used a cell phone to threaten a state senator and his family.
This month's indictment doesn't detail how much money guards at the McConnell Unit Prison allegedly received in exchange for the cell phones.
But in 2009, one top Texas investigator told CNN that a smuggled cell phone could fetch as much as $2,000.
In the McConnell Unit Prison, authorities said Wednesday that rampant corruption had festered since 2005.
Thirteen of the former guards arrested are charged with racketeering, and four face drug charges, Dodge said.
In total, authorities have arrested 29 people in connection with the alleged scheme. Three others are still at large, Dodge said.
Now, authorities said, some of the former correction officers who once guarded the Texas prison are behind bars themselves.