The detectives believed Isaac Smith.

The then-25-year-old voluntarily arrived at Allegheny County Police headquarters on Sept. 13, 2021 — hours after the body of a woman who he’d been seeing was found in the backyard of a house in McKeesport.

Smith told investigators he and Karli Short had been intimate, that she was pregnant, and she suspected the baby was his.

He also told them that if that turned out to be the case, he would remain in the child’s life.

During their three-hour interview, Smith denied any involvement in Short’s death, and told police he had arrived that evening to clear his name.

“We totally believed he was telling the truth,” Detective Mark Restori told a jury Wednesday.

But several weeks later, police charged Smith with Short’s murder.

His trial began on Tuesday before Allegheny County Common Pleas Judge Kevin G. Sasinoski and is expected to last two weeks.

During opening statements, the defense said that Smith is innocent.

The prosecution is seeking a conviction on first-degree murder for Short and her unborn child. She was shot once in the head in a yard off Furnace Alley in McKeesport in the early morning hours of Sept. 13, 2021. Short’s body was found later that morning by a neighbor.

She was five months pregnant.

In court on Wednesday, prosecutors played a video recording that showed Smith’s interview with Restori that evening.

The detective testified that Smith’s name came up early that day since Short thought he could be the baby’s father.

But the detectives never had to go looking for him.

“He found us,” Restori told the jury.

For about three hours that evening, the detectives talked to Smith.

Smith said he and Short had reconnected via social media after she reached out to him two years earlier.

Over that time, Smith said he and Short had been together three times, but that they were not in a relationship.

“I have a girlfriend now,” he said.

Smith said he didn’t tell his girlfriend about Short’s pregnancy.

“I was going to wait and see,” he said. “I was trying to handle it the best I could because I had never been in a position like this.”

It turned out, prosecutors said on Tuesday, that the baby was not Smith’s, but he didn’t know that at the time.

They told the jury in openings that Smith killed Short because he feared that news he had potentially gotten her pregnant would upend his life, including the relationship he had with a long-term girlfriend who had met his parents just the day before.

During his interview with detectives, Smith said he had been in Homestead the evening before the shooting visiting a friend’s new puppy, and then returned to his girlfriend’s house for the rest of the night.

He also told detectives that he texted Short at 8:42 a.m. that day and never heard back from her.

However, he said that as the day went on, he’d been hearing from people in the neighborhood that he was a suspect in Short’s death.

He went to the police, he said, to dispel the rumors.

As the interview was concluding, Smith asked the detectives if they would put it out to the community that his name had been cleared.

“I just don’t want to get extra grief,” he said. “Are you guys going to relay the message to the family that I came and cooperated?”

The detectives evaded the question but told Smith they understood why he was asking.

In addition to speaking with investigators, Smith also allowed them to make a copy of his cell phone’s contents and provided the vehicle identification number (VIN) for his Dodge pickup truck.

Restori told the jury that detectives reviewed license plate readers in the area of the shooting to see if they could track Smith’s vehicle that night, but they never spotted it.

However, Restori said they did recover video from around the time in question that showed a person walking toward the crime scene and then running away.

During Restori’s testimony, Deputy District Attorney Emma Schoedel asked Restori, “Did you believe the defendant was not involved in Ms. Smith’s murder?”

“I did,” the detective answered.

“Wholeheartedly?” Schoedel asked.

“Yes.”