The federal investigation predated the district attorney's investigation, during which authorities reportedly found $240,000 in cash in Gosnell's home.
He is already awaiting a March 2013 trial in state court, charged with murder in the 2009 death of a Virginia woman undergoing an abortion and with killing seven newborns by severing their spinal cords with scissors.
The grand jury likened the clinic to a "charnel house" with unsterilized equipment, bloodstained floors and a 15-year-old attendant who administered ultrasounds. Seven of the 10 defendants in the state case, including Gosnell's wife, have pleaded guilty. He pleaded not guilty.
Federal prosecutors said that in 2008 Gosnell began issuing prescriptions for oxycodone, Xanax and codeine, and said his customers were part of a "pain management" practice at his abortion clinic.
The feds said the number of prescriptions written for the drugs and filled at pharmacies increased from several hundred per month in 2008 to more than 2,300 in January 2010.
Authorities said that most of the customers seeking prescriptions for the drugs initially met with Gosnell but that they got only cursory or no physical exams.
The feds said Gosnell charged customers $115 for an initial office visit - later increased to $150 - with a follow-up visit fee of $50.
In July 2009, Gosnell began charging customers for refill prescriptions at the time of pickup, typically $20 for those paying in cash and $10 for those with insurance, the indictment said.
Authorities said West, Fluellen and Cross, and three others charged separately - Latosha R. Lewis, Earlene Tina Baldwin and Lynda Gail Williams - assisted Gosnell in running his so-called pill mill by taking customers' orders for refill prescriptions and selling the scripts.
A seventh former Gosnell employee, Steven Massof, was charged with dispensing drugs using a Drug Enforcement Administration registration issued to a physician. Massof was one of the defendants who pleaded guilty in the state case last month.
If convicted of all charges, Gosnell faces a mandatory minimum 20 years. West, Fluellen, Baldwin and Williams each face a mandatory one year in jail.