MINOCQUA - Evidence in a child sexual assault case was left untested for more than four years in a popular Northwoods vacation town, allowing a suspected molester to elude criminal charges until last month.

The case started in December 2013 when a mother reported to Minocqua police that her 2-year-old son might have been molested. Her son described feeling pain from an unnamed "doctor" who "put his belly up his butt," she said.

A nurse collected forensic samples from the toddler's body for DNA tests.

But according to case records obtained by USA TODAY NETWORK-Wisconsin, Minocqua police put the evidence in storage and ended their investigation after four days.

The case remained idle until state officials asked Minocqua police last year to submit the samples for DNA testing. The state had received millions in grants to pay for DNA tests and hoped the effort would help catch rapists.

The Minocqua case gained steam in June this year after the state reported finding an adult man's DNA on a swab of the toddler's body. Further analysis of the same swab reported finding the man's sperm.

On Nov. 30, Oneida County prosecutors charged the man, Brandon Darnick, 26, with first-degree sexual assault of a child. Darnick has pleaded not guilty, according to online court records. He could face decades in prison if convicted.

Darnick's lawyer, Brent DeBord, declined to comment on the case. As of Monday, Darnick was still in custody at the Oneida County Jail in Rhinelander.

In 2013, Minocqua police shelved the forensic samples that led to Darnick's charges due to a "lack of evidence" in the case, according to a police log. Minocqua police had called the toddler's complaints "difficult to understand" in 2013 and noted that a nurse had found no signs of injury or abuse.

Police also noted in 2013 that Darnick admitted to being alone with the boy in a bedroom — but other adults were in the home during the same period.

Minocqua Police Chief David Jaeger declined to comment on the case, which police incident reports show was investigated by Officer David Geiss in December 2013 but not reviewed by Jaeger until this year.

There is no indication in police reports that the case was reviewed by local prosecutors until Darnick's arrest last month. Oneida County District Attorney Michael Schiek did not respond to questions about his office's handling of the case.

Minocqua police arrested Darnick at his Arbor Vitae home. Police records say he denied assaulting the toddler, called the state's DNA test results impossible and suggested he had been set up by the toddler's mom, whom he knew in 2013.

USA TODAY NETWORK-Wisconsin normally does not identify victims of reported sexual assaults without their permission, so it is withholding the names of the boy and his mother.

The charges against Darnick mark the fourth known prosecution stemming from the state's efforts to test evidence in thousands of years-old sexual assault cases. The mountain of untested evidence had become commonly known as the rape kits backlog.

USA TODAY NETWORK-Wisconsin has chronicled the state's testing efforts since 2015, shedding light on individual cases and statewide patterns. In September 2017, the news network revealed that over 2,400 untested rape kits were collected through invasive forensic exams of children, such as the Minocqua toddler.

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Rape kits from children now account for three of the state's four prosecutions. In Waupaca, DNA from a 17-year-old girl's rape kit led to charges against a man previously convicted of sexually assaulting teen girls. In Beloit, DNA from a 13-year-old girl's rape kit led to new charges against an imprisoned rapist.

Unlike other states with strict DNA testing mandates, Wisconsin typically gives discretion to police agencies in sexual assault cases. The state requires testing only in certain cases with unknown suspects, but even there, USA TODAY NETWORK-Wisconsin has found hundreds of apparent failures to comply.

The Minocqua case may mark yet another failure. In 2013, Darnick and several other adults were described in police reports as involved in the investigation. Nobody was listed by police as a suspect, although they singled out Darnick in the investigation's findings for being alone with the toddler for a period.

It is unclear from Minocqua police records why the toddler's rape kit wasn't submitted for DNA testing until November 2017 — at least eight months after state authorities became aware of the evidence and flagged it for testing. It is also unclear why it took another year before Darnick was approached by police and arrested.

State Department of Justice officials, who oversee crime lab testing, declined to provide more details about how evidence was handled because the case is ongoing. While the state has recently seen growing delays in DNA analysis, the tests could have lingered this year because of police activities or other factors.

Minocqua police sought further tests of the toddler's rape kit but it is unclear from case records when that request was made and whether state DNA analysts performed the work. Most testing of old rape kits has been outsourced by the state to private labs and then reviewed by state analysts.

Minocqua, about 70 miles north of Wausau, is a popular tourist destination among Midwest families for its surrounding lakes and winter sports. Its residential population is about 4,500, according to state estimates.

Keegan Kyle is an investigative reporter for USA TODAY NETWORK-Wisconsin. He can be reached at kkyle@gannett.com or 619-206-5293.