Zimmermann, a UW student, was found murdered in her downtown apartment.
Brittany Zimmermann fought back.
She clawed and scratched at her attacker. She tried to escape. She called 911 screaming for help but the call got disconnected.
Despite her efforts, the 21-year-old UW student was beaten, strangled and stabbed to death in her downtown Doty Street apartment just after noon on April 2, 2008.
Since then, her family has suffered through numerous twists and turns in the case. They’ve waited as the years — and Brittany’s missed birthdays — have passed. Several tips and leads have come and gone as the gruesome murder has remained unsolved for more than a dozen years.
But with advancements in genetic-matching technology — which is utilizing DNA recovered from underneath Zimmermann’s fingernails and the clothing she wore when she was killed — authorities are now hopeful that the case is finally inching toward a conclusion in court.
In March 2020, the Dane County District Attorney’s office filed first degree intentional homicide charges against longtime suspect David Kahl.
Interviewed on the day of the murder, Kahl told police he was near Zimmermann’s house scamming college students. He admitted going door-to-door, asking for money to repair a car tire but instead using the cash to buy crack cocaine. While talking to police that day, he consented to a swab of his inner cheek to collect DNA.
Kahl, who has previous felony convictions dating back to the early ‘90s for sexual assault, theft and drug-related offenses, has steadfastly denied harming Zimmermann on numerous occasions over the years when contacted by police.
Now 55, he is serving a prison sentence at the Oshkosh Correctional Institution for his seventh drunk driving conviction. Though he’s scheduled for release in early November on that conviction, a $1 million cash bail has been imposed for the homicide charge, so it’s unlikely he’ll be freed before trial, which has yet to be scheduled.
In June, Kahl was found capable of assisting in his own defense after his attorneys requested a mental competency exam. At an arraignment hearing in July, Kahl stood mute when asked how he pleaded to the homicide charge; the judge entered a not guilty plea on his behalf.
As the years stretched on, Brittany Zimmermann’s murder “was never considered an inactive investigation,” says Capt. Daniel Nale of the Madison Police Department, who took over as lead investigator on the case in 2017. “We would follow up on tips, kept a list of the main players, and continued to have discussions with the district attorney’s office.”
While he declined to go into detail because of the upcoming trial, Nale tells Isthmus he interviewed Kahl in prison in early March 2020 just before the homicide charge was filed.
Nale was one of the detectives who helped canvass the Doty Street neighborhood looking for witnesses and clues on the day of the murder and expects to be called as a witness “several times” in the trial. He adds that DNA from the crime scene is a “very key” piece of the prosecution’s case against Kahl.
He believes that the evidence presented in the criminal complaint — in addition to what else is presented at trial — will result in a conviction.
According to the complaint, which runs 20 pages, six residents from Zimmermann’s neighborhood said Kahl asked them for money on the day of the murder. It also includes Kahl’s six different statements to police over the six years following the murder as well as the accounts of others — including Kahl’s mother — who said Kahl told them he was in Zimmermann’s apartment the day of the murder or that he knew who committed the murder or that he might be an accomplice to the crime.
Also included in the criminal complaint is a long list of DNA evidence sampled and analyzed over the years. The first DNA sample that matched Kahl as a “possible contributor” came from underneath Zimmermann’s finger nails. It was tested in 2011 by the Wisconsin State Crime Lab.
Starting in 2017, according to the complaint, law enforcement authorities began sending DNA samples from Zimmermann’s shirt and jeans to a Pittsburgh-based bioinformation company called Cybergenetics. “Using advanced mathematics, its computers translate DNA data into useful information,” the complaint says of Cybergenetics’ work.
Nale says it was a joint decision between police, the state crime lab and the district attorney’s office to use Cybergenetics for additional DNA testing. “To my knowledge, that was the first time MPD has used that company,” he says, adding that he can’t speak for the district attorney or state crime lab.
The complaint notes a match from Zimmermann’s shirt “and the defendant that is 226 thousand times more probable than a coincidental match to an unrelated Caucasian person. The chance of a false positive for this comparison is one in 12.7 million.” The complaint also lists a DNA match found between the bottom left leg of Zimmermann’s jeans and the defendant that was “18.7 million times more probable than a coincidental match to an unrelated Caucasian person.”
Another match from the jeans sampled in February 2020 was found by Cybergenetics to match so closely to Kahl’s DNA that “only 1 in 3.8 trillion people would match as strongly.”
Also included in the complaint is a letter that was sent to a Madison police detective in 2009 from an unidentified inmate at the Fox Lake Correctional Institution. The letter aims to pin the Zimmermann murder on a man listed in the complaint by the initials F.R. It states, “I have heard inmate [F.R.] talk about the murder of that girl [B.Z.] and how he and his friend’s [sic] killed her.” The state crime lab tested the envelope of the letter in 2019 and it was found to have a single source of DNA that matched to David Kahl.
Dane County District Attorney Ismael Ozanne declined two requests for interviews with prosecutors. Ozanne also did not reply to emailed questions asking about the challenges of prosecuting a 13-year-old case, the reliability of Cybergenetics DNA testing, or when this case may be resolved in court.
State public defenders also declined to talk about the case or upcoming trial. When reached for comment, Kahl attorney Benjamin Gonring said only that they “look forward to putting the government to its proof.”
DNA matching has been used to make or break cases since it was first used in criminal court in the late 1980s. Traditional DNA testing matches an individual to physical evidence, like blood. But TrueAllele, the DNA-matching technique patented by Cybergenetics, does not “match a suspect to physical evidence; it calculates the statistical likelihood that a person’s DNA is present in a complicated mixture of multiple people’s DNA, or in a minuscule amount of DNA left behind — for instance, after someone merely touched something,” as explained by Lauren Kirchner, who writes about the intersection of technology and government for The Markup.
Kate Judson, an attorney and the executive director of the Madison-based Center for Integrity in Forensic Sciences, has serious reservations about the technology used by Cybergenetics, which has refused — until recently forced by the courts — to provide access to the program’s source code.
“TrueAllele is a particularly controversial type of probabilistic genotyping software, because there’s been a real lack of transparency with it,” says Judson.
Judson points to a review released in June by the National Institute of Standards and Technology that found that there’s not enough publicly available data to independently assess the reliability of probabilistic genotyping software. “So, there’s not enough information out there to make an objective assessment of how good it is, how reliable it is,” she says.
Judson adds that there are many problems with relying on DNA evidence. It “won’t tell you somebody’s intent or whether they committed a crime,” she explains. “But you can place someone with an object by DNA testing.” And, DNA can be transferred. “If I use your pen and give it back to you, then you touch the pen. Now you have my DNA on your hands. Then, if you touch a doorknob, my DNA is now on that doorknob, even though I never touched it,” says Judson.
While the big numbers used to describe DNA matches in cases like this are alluring, Judson says there’s a strong chance they could be misinterpreted.
“They say that the chance of a false positive for this comparison is one in 12.7 million. That’s not really the way that the statistic works,” she says. “People believe that DNA is infallible and when law enforcement doesn’t properly understand what they’re talking about, it can cause a real problem.”
Isthmus contacted Cybergenetics via phone and email to ask about these criticisms. Dr. Mark Perlin, chief scientific and executive officer for the company, initially said he was available but then did not respond to attempts to schedule the interview.
Brittany’s mother, Jean Zimmermann, says no day goes by that doesn’t bring a memory of her daughter or a painful reminder that her life was so violently taken.
“We just had a family wedding for our niece. And we think, we’ll never have this. We will never ever be able to have our daughter get married,” says Jean. “Those are hard days. They’re all hard. Every day is hard.”
Her family has been anxious since the charges were filed. “Nothing is a given and nothing has ever been a given in this case,” Jean continues. “We all want it to be resolved and for it to be over. It’s been 13 and a half years. It’s gone on for far too long.”
She isn’t sure if she’ll attend the trial in person. “It depends on COVID levels,” she says, adding that Brittany, who grew up in Marshfield, was studying medical microbiology and immunology and wanted to be an infectious disease doctor. “This pandemic would have been right up her alley — she would have been helping.”
Next month there will be a pre-trial conference, where prosecutors and defense attorneys may explore resolutions before going to trial. That hearing is scheduled for Nov. 15, which would have been Brittany Zimmermann’s 35th birthday.
