For nearly three decades, Yvonne "Missy" Woods was Colorado’s star forensic scientist, relied on by police and prosecutors to test DNA evidence in the state’s most baffling crimes.

Her work was considered the gold standard by colleagues and helped put away infamous murderers, including the "Colorado Hammer Killer."

Then, in November, Woods abruptly resigned. The same day, the Colorado Bureau of Investigation said it had discovered anomalies in her work during an internal review and was launching a criminal probe.

The unfolding scandal—potentially one of the largest in the history of forensic DNA testing, according to experts—is throwing Colorado’s criminal justice system into chaos. The state said it would need to review and retest approximately 3,000 DNA samples that Woods handled. Public defenders estimate thousands of cases could be affected.

Prosecutors are bracing for numerous legal challenges from people charged or convicted based on Woods’s findings. State lawmakers recently allocated nearly $7.5 million for possible retrials and case reviews, along with the retesting.

At the center of the storm is a mystery: Was Woods just sloppy, or has she been purposefully cutting corners for decades to put people behind bars?

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"This is a huge, unprecedented mess," said George Brauchler, a former district attorney in the Denver suburbs whose office oversaw numerous cases in which Woods testified. "I want to know, what in the world did she do?"

Ryan Brackley, an attorney for Woods, said she is cooperating with the investigation.

"She continues to stand by the reliability and integrity of her work on matters that were filed in court, and particularly in cases in which she testified in court under oath," he said.

Investigators have released few details and haven’t said what prompted the review that uncovered anomalies in Woods’s work. They also haven’t said when their probe will be complete and whether criminal charges will be filed.

But in a Dec. 5 email to district attorneys across the state, the lab director of the Colorado Bureau of Investigation said Woods had in some cases altered data and in others, analyzed samples several times but reported only one result.

As of that date, investigators hadn’t found that any DNA evidence provided by Woods was inaccurate, according to the email.

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"CBI Forensic Services continues to review all cases worked by Ms. Woods during her 29-year career," lab director Shawn West wrote. "The CBI is utilizing all available resources to expedite this process."

As Colorado prosecutors pore over hundreds of cases that Woods worked on and await results of the investigation, they must consider two nightmare scenarios: Whether any of Woods’s cases ended in a wrongful conviction and whether some people correctly put behind bars must now be retried because of shoddy DNA testing.

"The impact to the confidence and integrity of the justice system—and the work that is going to be required on these cases—is really significant," said Michael Dougherty, Boulder County’s district attorney. "We will undoubtedly see defendants who’ve been convicted rightfully and justly trying to use this issue to their advantage."

His office has identified 56 closed cases and 13 open cases in which Woods was a witness or potential witness.

One current case he must contend with is that of Garrett Coughlin, whose trial for allegedly killing three people is set for April.

In November, the Colorado Bureau of Investigation alerted prosecutors that DNA testing conducted by Woods in the case was missing data.

Coughlin’s attorney Mary Claire Mulligan said she wants to know why the problems with Woods’s DNA analysis weren’t caught earlier, a question other defense lawyers have asked as well. Standard criminal-lab protocol requires a forensic scientist’s work on each case to be reviewed by a colleague.

"The fact that this could go on for 20-some years, and not once did it get caught by peer review, this says there is something very wrong with forensic testing in Colorado," Mulligan said. 

State law-enforcement officials have said the problems are limited to Woods and aren’t laboratorywide.

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But issues are popping up elsewhere in the state. On March 1, the Sheriff’s Office in Weld County, Colo., said it fired a veteran DNA analyst and will pursue criminal charges against her after anomalies in her work were discovered during the Woods investigation.

A sheriff’s spokeswoman said that while the two inquiries were separate, the state crime lab where Woods worked and the one where this DNA analyst worked have teamed up on cases.  

It was through solving cold murder cases that Woods, 60 years old, earned a reputation as a skilled criminal scientist. She was adept at finding the faintest DNA samples on old crime-scene evidence collected years before the advent of advanced forensics, including clothing and lip balm, according to court documents.

One of her most notable cases dates back to 1984, when Bruce and Debra Bennett and their 7-year-old daughter Melissa were found beaten to death with a hammer inside their Aurora, Colo., home. Another woman, Patricia Smith, was raped and fatally beaten with a hammer in her Denver suburban home around the same time.

The identity of the Hammer Killer was unknown until 2018, when Woods helped identify a suspect based on evidence extracted from the crime scenes. The DNA profile matched a man named Alex Ewing, who was already serving a decadeslong prison sentence in Nevada for attempted murder in a pair of bludgeoning attacks.

With the help of Woods’s testimony, Ewing, who pleaded not guilty, was convicted of the four killings and given four life sentences. He is appealing one of the convictions, and his attorney said the investigation into Woods bolsters their case.

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"It really raises significant questions as to the reliability of the DNA evidence, which was the primary evidence that convicted my client," said lawyer Suzan Trinh Almony. The district attorney’s office in Jefferson County, which prosecuted Ewing in the murder case that he is appealing, said it couldn’t comment because it has also been assigned to prosecute Woods if criminal charges are filed.

Brackley, Woods’s attorney, declined to comment on any specific cases, but said Woods’s work was "beyond reproach."

Woods has already been named in at least one lawsuit related to her work.

Days after she resigned, attorney Mark Burton filed a federal suit alleging that Woods’s faulty analysis helped put his client James Hunter behind bars.

Hunter was arrested in 2002 after a woman and her 5-year-old daughter living in a Lakewood, Colo., trailer park were sexually assaulted by a man with a sock over his face. The woman said the man’s voice sounded like Hunter.

Woods conducted a microscopic hair comparison that she said placed Hunter at the scene of the crime, according to the suit. But at the request of defense attorneys, the hairs were then tested by an outside lab, which determined they belonged to the victim.

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When the error was revealed, Woods told the judge that it was a "blow to her ego," according to the suit. The case was dismissed.

Then, 10 months later, detectives said they had located another hair. Woods tested it and found it contained DNA from Hunter. He was arrested again and eventually convicted of sexual assault and burglary. He is still in prison today.

"Once we heard about Missy Woods, it all made sense," Burton, Hunter’s attorney, said.