SUGAR LAND, Texas (AP) — Nearly 100 unmarked graves near Houston believed to contain the century-old remains of imprisoned African-Americans will be exhumed after workers building a school discovered the gravesites.

A judge on Monday gave the Fort Bend Independent School District permission to begin a monthslong process, at a cost of upward of $1 million, to exhume the graves.

Bones were found this year as the ground in Sugar Land was being prepped for a $59 million career and technical center for the district's students.

The area is near the Old Imperial Farm Cemetery, which contains 31 marked graves for primarily white guards and prisoners on land once part of the Imperial State Prison Farm.

University of Houston anthropology professor Kenneth Brown said someone with a specialized knowledge of African-American history needs to examine the remains. Experts must determine the demography of those buried there, he said, but that step and other fundamental ones were interrupted as the school district chose to continue with construction work on other parts of the land.

"Part of my concern is: Why in the world did it take so long to find those graves?" asked Brown.

Cemetery guardian Reginald Moore, who has argued for years that the area was the burial site for unnamed black prisoners who died under harsh working conditions in the years after the Civil War, has criticized the school district for moving ahead with plans to build there.

"These people were used, abused, neglected and taken advantage of without any recognition," Moore previously told the Houston Chronicle. "I feel like it was an atrocity and somebody had to speak up for them."

District spokeswoman Veronica Sopher said school officials share Moore's concerns and want to determine who was buried there and under what circumstances.

"Obviously as an educational institution we want to use this as a learning opportunity, not only for our students but for our community," Sopher said Tuesday, later adding, "It's important for us that this process be handled in the most dignified manner possible."

The district is planning to keep its timetable for the center to open in fall 2019, she said.

The area is known historically for its sugar cane crops and sugar mill that gave rise to the Imperial Sugar Co., based in Sugar Land. The prison system in the decades after the emancipation of slaves in 1865 leased out black prisoners to plantation owners and others, according to the Texas Historical Commission.

The Imperial State Prison Farm and other Texas prisons were known for their harsh treatment of prisoners and living conditions before public outcry brought about reforms in the early 1900s, according to the commission.

Brown said one of the things that must be determined about the unmarked graves is whether they were part of a long-forgotten plantation cemetery or a prison cemetery.

"The convict labor system was heavily geared to black workers," he said. "They were put to work on sugar cane because that's what they had previously been doing."