DNA exists in every cell of our bodies, and its unique genetic sequences make each person’s DNA distinct. In forensic science, scientists analyze DNA samples to help solve cases, particularly using short tandem repeats (STRs) in non-coding DNA for identification.

The DNA department at the Hennepin County Forensic Laboratory in Minnesota experienced a 79-day shutdown due to an unexpected issue. The culprit? A plastic plate used for DNA analysis was found to be contaminated, compromising the lab’s results and halting operations. This issue didn’t just affect this one lab—it had implications for several other forensic labs as well.

The plastic plate in question resembled a rectangular plastic brick with 96 small wells designed to hold swabs containing DNA samples. The lab processes thousands of DNA samples annually, necessitating a large supply of these plates. They were purchased in bulk from a manufacturer and delivered to the lab for collecting crime scene samples. To ensure accuracy, each analysis included a negative control (a sample without DNA) and a positive control (a sample with known DNA). The lab discovered anomalies in the DNA profiles of the negative controls, and further investigation traced the contamination back to these plastic plates.

After multiple experiments, scientists confirmed that the contamination wasn’t due to lab personnel or sample handling but originated from the plates themselves. What made the situation even more perplexing was that the same DNA profile repeatedly appeared in multiple cases but did not match any known criminals, making it even harder to trace the source of contamination. The lab began re-examining all DNA samples processed with these plates since 2016, reviewing about 50,000 DNA records to identify potentially contaminated cases.

Other labs, both domestic and international, reported similar issues, indicating that this contamination problem might have affected case analyses across multiple regions. Consequently, the Hennepin County lab decided to suspend routine casework, re-examine past cases, and retest those potentially impacted by the contamination.

As for how the plates became contaminated, one theory suggests that during manufacturing, a worker’s sneeze might have accidentally introduced DNA onto the plastic before it was molded. While there’s no definitive answer yet, the lab has ceased working with the supplier of these plates and plans to purchase new DNA analysis equipment to prevent similar issues in the future.

This incident not only exposed potential risks within forensic laboratories but also highlighted the sensitivity and complexity of lab operations. To minimize the impact, the lab maintained close communication with prosecutors and defense attorneys and provided transparent solutions to the public. For unresolved cases, the forensic lab recommended retesting to ensure justice is served.

For cases that have already been closed, if contamination influenced a conviction, defense attorneys may pursue further legal action. This entire process will require substantial time and resources, including acquiring new equipment and materials, to ensure future DNA testing is more precise and reliable.