A Montgomery County man sexually assaulted and killed his 10-month-old daughter in the fall 2020, prosecutors said Monday at the start of his trial on murder charges.

But Austin Stevens’ defense attorney told jurors that prosecutors had built their case on “suspicions, speculation, and guesswork.”

Stevens, 31, is charged with murder, involuntary deviate sexual intercourse, endangering the welfare of a child, and related offenses in the death of his 10-month-old daughter, Zara.

Assistant District Attorney Brianna Ringwood said Stevens assaulted the girl “in secret and in isolation.”

“You might not want to believe it, he tried to hide it, but that’s simply what the evidence shows,” Ringwood said.

Stevens’ attorney, Evan J. Kelly, disputed Ringwood’s statements, saying there were no witnesses to the alleged assault and murder. He urged the jurors to acquit Stevens, who he said had no motive to kill his daughter.

“It’s easy to hear a powerful opening statement and it’s easy to rush to judgment,” Kelly said. “Please be patient and listen to the evidence.”

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Police were called to Stevens’ apartment in Collegeville in October 2020, where they found Zara unresponsive, according to the affidavit of probable cause for Stevens’ arrest. She had no pulse and was not breathing. A medic pronounced her dead at the scene.

Officers who responded to the scene said Stevens was “unusually calm” after the child’s death, according to Ringwood. Stevens, a former coach of the Lower Providence Warriors youth football team, had a custody agreement with the baby’s mother, and had picked up the baby hours before her death.

The Montgomery County coroner later ruled that Zara died from blunt-force trauma to her head, and that her death was a homicide. The girl also had extensive bleeding in her brain, and doctors remarked that her injuries were consistent with her being shaken violently and hit against a hard surface at least two times. They had seen similar wounds, they wrote in court filings, in victims of high-speed car crashes.

Zara also bore the signs of sexual assault, including bleeding and soft tissue damage, according to the coroner.

In an interview with county detectives, Stevens said he had left the baby in the bathtub briefly to get a beer from the kitchen, and suddenly heard a bang, the affidavit said.

But text records from Stevens’ cell phone showed that he waited more than an hour to call 911. Instead, he sent flirtatious text messages to a woman, telling her how badly he wanted to see her, according to prosecutors. He also made several Google searches, including “what if baby stops breathing,” and “how do you know if baby is dead,” Ringwood told jurors.

She asserted that Stevens concocted the story to “cover up what he did to that 10-month-old baby.”

“But despite the defendant’s best efforts to hide the truth, the truth was revealed through this investigation,” she said.

The trial, before Common Pleas Court Judge William Carpenter, is expected to last through Friday.