After discovering that hundreds of DUI cases from the first half of 2016 could have been affected by improper calibrations, Philadelphia police said Thursday that their breath test machines have been recalibrated and returned to service.

Joseph Kelley, the private attorney who notified the Philadelphia police department of the issue Wednesday, said that between 500 and 1,000 DUI cases are estimated to have been affected by the improper calibrations, the Associated Press reported, adding that Philadelphia police also had wrongly calibrated the machines in 2011.

“Earlier this year our instruments were tested, but due to human error, an expired solution was used during that test,” police spokesman Lt. John Stanford said in a statement, adding that all of the instruments were immediately removed from service before they were retested with a new solution and returned to service on the same day.

According to state guidelines, the authorities have to calibrate the machines annually with updated solutions that determine the accuracy of blood alcohol readings.

“Although the solution was expired, the results calculated properly and accurately for each instrument,” Stanford said. “While the expired solution appears to have had no impact on the readings calculated by the instruments, the department takes this very seriously.”

Stanford also stated that the department has asked the district attorney's office to review cases involving breath tests from Jan. 17 to June 29.

“While no organization is proud of errors, we do appreciate the attorney bringing this to our attention. Optimum service to our citizens is not an option; therefore, we have quickly rectified this matter and we remain committed to exceeding minimum standards set by policing guidelines,” Stanford said.