A Texas prison supervisor has resigned and four guards have been fired amid allegations they orchestrated the planting of two screwdrivers in an inmate's cell.

Texas Department of Criminal Justice spokesman Jeremy Desel told the Houston Chronicle that the planted evidence "appears to be an isolated incident" at the W.F. Ramsey Unit in Brazoria County. Desel said the scheme began with one supervisor but that all parties involved didn't show integrity.

The newspaper obtained copies last month of an email from Capt. Reginald Gilbert that ordered a quota for officers to write inmate disciplinary reports. Gilbert said any sergeants who missed their quota would face disciplinary consequences.

The email said the new policy was effective March 10, 2018, but prison officials abandoned the quota system weeks later. The department launched an investigation after the newspaper's reports.

Desel said Monday that the evidence-planting investigation is unrelated to the quota system.

An inmate's mother sent a letter to the department's Office of the Inspector General on May 25, alleging that her son had been set up by guards at the Ramsey Unit. The agency began investigating and determined the mother was likely correct, Desel said.

Major Juan Jackson resigned under investigation. Gilbert, who wrote the email ordering a disciplinary report quota, was later demoted, Desel said.

Jennifer Erschabek of Texas Inmate Families Association said the departures confirm their suspicions.

"We've been claiming in the past that family members are being set up with these cases, but it's been so hard to prove — and we finally have the proof," she said.

The Office of the Inspector General's investigation into the matter is ongoing. The state's criminal justice agency is also conducting an internal investigation into the Ramsey Unit and its prison culture.