How a piece of plastic shut down DNA testing at the Hennepin County crime lab

It was technically a successful scientific discovery when analysts at the Hennepin County forensic science lab recognized there was a contaminant somewhere in its DNA process. That did not lessen the sense of dread.

Allison King, the DNA supervisor and technical leader, summed up the general feeling inside the lab when the problem was discovered: "Oh, God, no."

It would take a herculean effort by lab scientists to identify a rectangular piece of plastic that is used routinely in DNA analysis around the world as the source of contamination. Then they had to reanalyze 50,000 DNA profiles to identify a handful of cases which may have been affected, and take corrective actions to get back up and running last week. The shutdown of the DNA section of the crime lab lasted 79 days.

The lab is a compact, labyrinth facility in the shadow of U.S. Bank Stadium in downtown Minneapolis. It works with dozens of law enforcement agencies throughout Hennepin County with the exception of the Minneapolis Police Department, which has its own forensics division. The lab examines and analyzes crime scene evidence like fingerprints, firearms, ballistics and DNA — either through evidence collected by its own crime scene investigators or delivered by law enforcement agencies.

The piece of plastic that caused the contamination is called a plate, but it's more like a brick.

It's rectangular with 96 holes in the shape of tiny pipettes where DNA swabs are inserted for testing. The swabs are like long Q-tips, similar to what's used to test for COVID-19, only these are used to match genetic profiles from evidence like blood droplets near a broken window or residue left on steering wheels from car thefts. King estimated her office processes about 10,000 DNA samples per year.

That means it needs a lot of the plastic bricks. They are shipped to the lab in boxes of 10.

When the lab runs a test there are samples from multiple crime scenes.

"We batch cases, otherwise we would be really inefficient," King said. "We might have three samples from a burglary, four or five samples from a robbery, a few samples from a felon in possession of a firearm."

It makes for intensive bookkeeping. There is also a control in every sample test. The control can be negative or positive, but if the control doesn't match the lab is alerted to a possible contaminant. That's what led the DNA section of the lab to shutdown on Sept. 17 after a negative control came back with a DNA profile.

"We started to investigate where it came from and we saw it repeated multiple times," King said.

Perhaps the most confusing part of tracking the contamination was this: The DNA profile was flagged as a contaminant because it was in the negative control — a space where there was not supposed to be a DNA profile — but analysts in the forensic lab started searching their previous casework and found the DNA profile in several cases.

The profile dated back years but always appeared at random and did not match any known criminal in Hennepin County's database or in national or international databases.

The scientists tried an experiment. They ran the plastic brick with just liquid — no samples — to see if it would come back positive for the same DNA profile. It did. They ran the test four times and the profile came back every time. In that moment they realized the contamination was not from someone inside the lab or from mishandling some kind of evidence, but instead was inside the plastic.

And it was not only inside the Hennepin County forensic lab. After they reported the contamination, several DNA labs across the country and one internationally said they had seen the profile before in their casework.

"We just happened to be the ones who discovered it," King said, adding, with an exhausted nod of her head, "Super cool."

There are hypotheticals as to how this happened — maybe an employee at the manufacturing plant sneezed into a vat of unmolded plastic that was then shaped into the bricks and sent out to labs — but no real certainty.

Once the lab identified the problem, the real work began.

"Every single one of my analysts was not doing casework so we re-examined all DNA profiles that we have developed since 2016 when we started using this product, by hand," King said. "They deserve raises."

The forensic lab views itself as an independent entity in the criminal justice system with its scientists having no opinion on how their work determines the outcome of a case. They first notified all of the law enforcement agencies they work with about the contamination. They have also been in close contact with the Hennepin County Attorney's Office and the Hennepin County Public Defender's Office. In the end, the analysts recommended fewer than 10 cases they believe should be retested because the contamination potentially put their initial DNA analysis in question.

So far, all sides are saying the lab has operated with the utmost transparency.

Ginny Barron, who runs the Barron Law Office and reviews DNA cases for the Public Defender's Office, said the crime lab went "above and beyond" in terms of communication but there will be next steps for defense attorneys. Barron said the crime lab plans to supply a corrective action by the end of the year that includes lists of all open or closed cases that have been affected by the contamination — a list that will extend beyond the fewer than 10 cases the lab has identified as needing retesting.

"If those cases are open we can litigate where appropriate," Barron said. "If they're closed we would have to look at what, if any, postconviction actions there would be if that's appropriate."

Just because the issue was identified didn't mean everything was immediately resolved.

It ended the lab's relationship with the previous vendor that supplied the plastic bricks and DNA analysis machine. "We feel strongly that they're not meeting the expectations of somebody we need to work with," King said.

It also created expenditures. In November, the Hennepin County Board approved a $223,000 contract with the Life Technologies Corp. for new DNA testing equipment and materials for the crime lab. The major piece of equipment that was acquired is still being tested. It's called the Applied Biosystems HID Nimbus Presto System and it advertises the ability to process the most challenging samples including "bone, tooth and touch DNA."

The company will also provide new plastic bricks.