YAMHILL COUNTY, Ore. — Unspecified clues led searchers to a heavily wooded area where they found the hidden bodies of a Salem mother and her 3-year-old son, Yamhill County District Attorney Brad Berry said during a Monday press conference.

Karissa Alyn Fretwell, 25, and her son, William "Billy" Fretwell, were last seen on May 13, and reported missing several days later. Michael Wolfe, the boy's father, has been arrested and charged with two counts of aggravated murder and kidnapping.

After two hours of searching, they were found Saturday about 10 miles west of Yamhill, Berry said. The bodies were covered with debris and partially hidden, he said.

The bodies were positively identified on Sunday by the Oregon State Medical Examiner as Karissa and Billy Fretwell.

Karissa's cause of death was homicide by a single gunshot wound to the head. The cause and manner of death for Billy has yet to be determined. That information will be released after further testing.

"I'm pretty confident it will be a homicide," Berry said Monday. "But the medical examiner could not say [Sunday] what the exact cause and manner [of death] was yet, without the further tests."

Berry said Monday that a firearm has been discovered, though he couldn't confirm if it was the gun used to kill Karissa Fretwell.

The area where the bodies were found is private property owned by Weyerhaeuser, one of the world's largest private owners of timberlands. Berry said Wolfe is familiar with the area.

"He recreations up there, like fishing, things like that. There are other areas within the Weyerhaeuser property that we believe he's actually had permits before to cut firewood up in that area. We were aware that he was familiar with the area," Berry said.

WATCH: Briefing on Fretwell case

The area had been searched before, during the past two weeks, before the bodies were discovered Saturday. The search effort Saturday was larger than previous searches in the area, with 40-50 people searching.

Reporters asked Berry if investigators had received a single tip, from Wolfe or someone else, that helped lead them to where the bodies were hidden. Berry said only that it was comprehensive detective work that narrowed the search.

Berry said the discovery of the bodies is a really hard thing for investigators, but that it will help the case against Wolfe.

"This was really tough on investigators out there. They've been working relentlessly to find these two. It was a really hard discovery that they made," Berry said.

"As far as the prosecution and the case getting put together, it's a very different type of prosecution where we actually have the bodies than if we didn't. I felt we had sufficient evidence to endorse him being arrested on it and to approach a grand jury on this, but certainly without the bodies, the case was a very different case, a tougher case moving forward," he said.

Grand jury indicts Wolfe on aggravated murder

A grand jury in Yamhill County indicted Wolfe last week on two counts of aggravated murder and two counts of kidnapping, in addition to a charge of aggravated murder constituting domestic violence.

Berry said during Monday's press conference that close to 20 search warrants have been obtained and executed.

Wolfe will be arraigned in court on June 21 at 1:30 p.m., Berry said Monday.

RELATED: 'Sufficient evidence' to confirm missing Salem mother, 3-year-old son were killed, Yamhill Co. DA says

Just two months ago, Wolfe was ordered in April to pay roughly $900 a month in child support, according to court records.

Court documents say detectives met with William's babysitter, who said Karissa confided in her that Wolfe and his wife had previously threatened Karissa and told her they were going to take William and get custody of him.

Aggravated murder carries the harshest punishments in Oregon and makes someone eligible for the death penalty. However, there is currently a moratorium on the death penalty in Oregon, so the likelihood of anybody on Oregon's death row being executed is close to zero.

Detectives tracked Wolfe’s location with cellphone tower records

Authorities interviewed Michael Wolfe twice, on May 18 and May 20, and Wolfe told detectives he’d had no recent contact with Karissa since their April court appearance, according to court documents.

Cellphone records, however, showed Karissa’s phone sent a text message using a cell tower 1.4 miles southeast of Gaston, Ore. on May 14, documents said. The area includes Wolfe’s property at 2700 Northwest Phillips Road, where authorities focused their search.

RELATED: Search continues for missing Salem mother, son after father arrested

Authorities have not said what the text message said or who received it.

Detectives also obtained location information that placed Karissa’s phone near Cascade Steel, Michael’s place of work, in McMinnville on May 14.

Surveillance video from Cascade Steel showed Wolfe leaving the area where he works on a golf cart around 8:45 p.m. on May 13 and returning around 2:35 a.m. on May 14 holding a white trash bag, according to Yamhill County court records.

Detectives tracked Wolfe’s cellphone using cell tower data and found that his phone began using towers indicating he was moving south, away from Cascade Steel, around 8:55 p.m. on May 13. A few minutes later, his phone used a tower covering parts of Salem, including the area where Karissa lives, court documents said.

His phone remained in the Salem area until around 1:38 a.m on May 14, when cell tower data indicated Wolfe began moving north. Around 2:12 a.m., his phone used a tower near Cascade Steel, where he was seen on surveillance video returning to his work area about 20 minutes later.

Records showed Wolfe’s phone was using a tower near his home around 4:34 a.m.

According to court documents, detectives believe Karissa’s cellphone and Wolfe’s cellphone were in the same approximate places during the night of May 13 and the early morning of May 14.

Berry said Monday that investigators know Karissa and Billy were kidnapped on the night of May 13 and into the early morning hours of May 14, and they're confident both were deceased by May 16.