GRAND FORKS — Before Brynley Rymner died, a Grand Forks doctor called her injuries some of the worst he had ever seen.

One of the last pictures of the 5-month-old shows her on life support at Sanford Medical Center in Fargo with injuries investigators believe she sustained when her mother's live-in fiance shook her when she wouldn't stop crying.

While it didn't take police long to identify the suspect — the then 18-year-old Mason Kamrowski — they did not know what led up to the baby's death until right before he was sentenced. Nor did family members, who expressed frustration at the sentencing hearing.

But according to newly-released information from the Grand Forks Police Department, Kamrowski had to explain to investigators what happened as part of his plea agreement.

He gives his account of what happened in a 13-minute recording provided by Grand Forks police.

Interview Audio:

Warning: Some may find parts of this confession graphic.

On May 21, 2018, Mason Kamrowski said he returned home to his south side Grand Forks apartment after a day of work. His fiancee — Jayden Rymer — asked him to watch her daughter so she could go grocery shopping. Kamrowski said the baby started getting fussy when he got out of the shower.

"So I picked her up and walked around with her, and she kind of got a little tired so I laid her in her crib, and I finished getting dressed and laid down for a little bit," he explains.

But the baby continued to cry. Kamrowski said that's when he snapped and shook her. In the recording, he tells the detective that he grabbed her around the ribs.

"It wasn't violent," Kamrowski said. "I mean, I wouldn't say it wasn't long; it was just a couple of times."

Kamrowski then tells the detective he didn't see the baby's head "flopping back and forth" when he shook her, though doctors strongly disagreed.

The detective then tells Kamrowski that a doctor said the girl's injuries were some of the "worst he's ever seen."

"Oh wow," Kamrowski responds.

The detective then asks what happened after Kamrowski shook the baby.

"She stopped moving; that's when I kind of freaked out," Kamrowski says, explaining that he then started to scream.

Instead of calling for an ambulance, Kamrowski then called his finance's sister and they rushed Brynley to Altru Hospital in Grand Forks. The baby was then was flown to Sanford in Fargo. After several heart attacks and strokes, she died the next day.

Kamrowski said he was stressed out the day he shook the 5-month-old.

"I don't just want to blame it on work, but I'm tired, (it) was a long day ... and me and Jayden argued a lot, you know what I mean?" he said. "(I had) a lot of stress built up and it all kind of just balled up at once."

He also expressed remorse about what happened.

"(I) feel terrible about it, and there isn't a day that goes by I don't think about it," Kamrowski said. "If I could trade places with her in a heartbeat I would ... I can't live with myself for it."

Rymer's family told WDAY they do not believe Kamrowski is telling the truth and believe he did more to hurt the baby.