Heeringa went missing on April 26, 2013, from her job at a gas station in Norton Shores. The 25-year-old mother was never seen again and is presumed dead even though her body has never been found.
Testimony on Friday came from Gerald McCarthy, a computer crimes expert with Michigan State Police. McCarthy says he found 15,000 videos of high school girls at swim meets on Willis' computer.
The videos were believed to have been made by Willis in the years leading up to his arrest in May 2016.
McCarthy says there's data that shows some of the video were recorded with the camera found inside Willis' silver van. Those videos were saved in a folder titled "homemade."
All of the videos are believed to be from high school pools or public pools in Muskegon, Ottawa and Kent counties. And the videos also, "were taken with a special filter that made it look like you were looking through the girls' swimsuits," McCarthy testified.
All of the video were taken while the female competitors were outside the swimming pool. And while the female competitors were getting ready to enter or exiting the pool.
"The focus of the videos were all on the breasts, buttocks and their groin area," said McCarthy.
Investigators say Willis had no reason to attend or had any connection to the swimming events.
Muskegon County Prosecutor D.J. Hilson is expected to call a few more witness to testify starting Tuesday morning.
One of those witnesses is expected to be the lead Norton Shores detective on the Heeringa investigation.
The case could go to the jury late Tuesday, but that is more likely to happen sometime on Wednesday or Thursday. Jurors are expected to begin their deliberations later this week.