LOS ANGELES – The Los Angeles County Sheriff’s Department said this week it will retest 4,000 DNA samples and open an internal investigation after learning that it used potentially flawed test kits for eight months.

Sheriff’s officials said Wednesday that a test kit manufacturer sent a letter in August warning the department to stop using certain kits that were prone to giving incomplete results. However, the letter was received by a civilian employee who didn’t discard the kits or send them back, according to a department statement.

The department used the flawed kits from July through February, testing thousands of samples from criminal investigations.

The problem was discovered Monday when a supervisor at the department’s Scientific Services Bureau found the manufacturer’s letter.

The department said it has opened an internal investigation to assess how much the faulty kits have affected criminal cases, and will retest some 4,000 DNA samples.

“We take the integrity of our criminal investigations and the reliability of our forensic testing very seriously,” Sheriff Robert Luna said in a statement. “The Sheriff’s Department is working diligently to assess the impact and to prevent such situations from occurring again.”

Los Angeles County District Attorney Nathan Hochman said his office had begun working with the sheriff’s department to determine the extent of the problem.

Sheriff’s officials said the bad tests might have led to incomplete results, but they are “not likely to have falsely identified any individual.”

The department declined to name the manufacturer.