The first remains were discovered in February in Sugar Land, a suburb southwest of Houston and now authorities have learned who these people were freed Black people forced to work in convict labor camps.

For over a century, these graves were underground and untouched. But the finding that they likely held the remains of slaves, which researchers announced Monday, highlights an era that’s largely forgotten in history at a time when slavery was illegal, but many Blacks were essentially still enslaved.

The Sugar Land property is owned by the Fort Bend Independent School District, which is building its new technical school on the land.

The bodies were each buried in individual wooden caskets. Of those analyzed so far, all but one are men. Researchers say they could have been as young as 14 and as old as 70. Despite the passage of time, researchers can tell that the workers were malnourished or sick and faced huge physical stress when they were alive.

Each time a body is exhumed, it takes up to two days, plus up to eight hours of cleaning and up to 15 hours for reviews of the test results. So far, they’ve dug up 50 graves and analyzed more than 22.

Once they’re dug up, a team of forensic archaeologists will look for more information on the corpses, such as their medical conditions and how they died. After that, the school district will work with the state’s historical commission to figure out where to rebury them.