Federal authorities located at least 18 undocumented workers on Monday and said they could face deportation.
"These defendants ruthlessly exploited their immigrant employees, stealing their wages and requiring them to live in unregulated boarding houses, in effect creating a modern-day plantation system," Lynch said.
The federal probe has widened to 40 locations in seven states, including Florida, Illinois, Maryland, Michigan, New Jersey, Pennsylvania and New York.
A spokeswoman for 7-Eleven previously said it "will take aggressive actions to audit the employment status of all its franchisees' employees" and pledged full cooperation with the federal probe.
After initial court appearances by eight of the nine defendants, they were ordered to be held without bail. Their next court appearances are scheduled for July 15.