MARYSVILLE — A Marysville baby suffered brain damage and blindness that will likely be permanent, when her father allegedly assaulted her, waited a day to get medical care and tried to cover up the beating with a story about falling down stairs, according to police reports filed in court Wednesday.

Kenneth Scott Nason, 32, told police he’d been holding his 4-month-old child in his arms around 1 p.m. March 10, when he slipped on a job application that was left on the stairs at a Marysville home.

According to Nason’s story, he’d been wearing long sweatpants that came over his heel, and when he fell the child flew into the air and struck her head against a wall.

Police found the story to be full of conflicting details and inconsistent with the baby’s injuries. Nason was booked into the Snohomish County Jail on Wednesday for investigation of first-degree assault of a child.

The mother left for work around 9 a.m. March 10. On a break she returned a phone call from Nason. He told her he’d been going downstairs with the baby when he slipped. He said the child had a scratch on her face, but otherwise she was acting normal.

Later, in a police interview, he recalled the girl was crying and gasping, and her face was red, swollen and bruised.

The girl’s mother came home around midnight. She gave the girl a bath, but didn’t notice a broken blood vessel in her eye until the morning. By then the child was vomiting. The mother took her to the Swedish hospital in Edmonds by bus March 11.

Doctors found bleeding in three different spots in the baby’s brain. She suffered “terrible, terrible retinal hemorrhages” and a shearing hemorrhage in the brain that doctors typically see in high-speed crashes, according to a police summary of the diagnosis.

The mother told detectives Nason regularly grew frustrated with the child because she cried often. She reported Nason used heroin in the past. She suspected him of using again, court papers said.

A detective recorded an interview with the father at the hospital. He claimed the marks on the girl’s head — an imprint in the shape of a hand — came from the trim of the stairs, according to police. He agreed to let officers come to the home so he could show them what happened. They noted there was no trim on the stairwell. Nason reportedly changed his story. He claimed the marks came from him grabbing the girl’s head, to protect her as they fell.

Nason re-enacted the fall on video. He cradled a doll with the head to the right his body. Earlier he’d said her head was to the left of him, and the girl flew in the air across his body, according to police. In the first interview, he’d reportedly said he slipped on the second stair. The second time, he said he was at the very top of the stairs.

The girl was transferred to Harborview Medical Center. Days later police asked the mother about the job application. She said she’d picked it up from a pizza place and filled out part of it for Nason. He claimed he’d turned it in, but she’d found it on the floor, according to her account. They argued about it. Afterward she’d folded it up and put it on a coffee table, she told police. Officers noted the application was still on the coffee table when they visited the home, and that no one else saw it on the stairs that day.

Police showed the re-enactment video to hospital staff.

“The medical team was unanimous in dismissing, as a realistic possibility, the defendant’s explanation for this child’s injuries,” deputy prosecutor Matt Baldock said in court Wednesday.

Detectives arrested Nason for the alleged assault and warrants in past domestic violence cases. A judge set bail at $250,000 in Everett District Court.

Court records show a different girlfriend reported Nason choked her, threw things at her and threatened to push her off a balcony in 2016. She stated he forced a light bulb onto her arm and left her with second-degree burns. She reported he’d harmed their toddler son, too.

“He shoved me from behind, causing (her son’s) head to be smacked in the wall,” the ex-girlfriend wrote in court papers.

She wrote that when police were called Nason said the boy tripped.

Caleb Hutton: 425-339-3454; chutton@heraldnet.com. Twitter: @snocaleb.