Iowa Gunman Who Killed Family on Camping Trip Had Never Met Victims Before Shooting: 'Completely Random'

The gunman who killed three members of an Iowa family at a campground last Friday had never met the victims before opening fire, investigators say.

Tyler and Sarah Schmidt, both 42, and their 6-year-old daughter Lula, were shot dead in their tent while camping at the Maquoketa Caves State Park in Jackson County, Iowa, Friday morning.

Lula's brother, 9-year-old Arlo, was the only family member to survive the attack.

Following the murders, the gunman, 23-year-old Anthony Sherwin of La Vista, Neb., turned the gun on himself, PEOPLE previously reported.

Citing agents with Iowa's Division of Criminal Investigations, WHBF-TV reports Sherwin was also camping with his parents at the same campground, 75 yards away from the Schmidts. However, Sherwin and the victims had no interaction with each other prior to the shooting.

"All we know is that this was completely random, nobody in either's family knew anybody," Sarah's brother, Adam Morehouse, told the outlet Tuesday. "We didn't know the suspect; the suspect didn't know us."

"We don't know of any interaction that occurred. It was simply this individual woke up that morning and decided to pick a tent and walk into that tent, where my sister and her family were sleeping, and never got a chance to wake up," Morehouse said.

Investigators are still working to uncover a motive in the slaying of the Cedar Hills, Iowa, family.

"At this time, we have not been able to come across any type of interaction between him and the Schmidt family, nothing that precipitated it as far as we know," Iowa DCI Assistant Director Mitch Mortvedt said, per the station.

After interviewing the shooter's family and other campers, Mortvedt noted there was "nothing that we can establish that precipitated anything, other than just the randomness of it, the proximity."

According to the station, Mortvedt would not provide details on how Arlo made it out alive.

"It's tough," Mortvedt told WHBF. "So heartbreaking on every level."

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Speaking with the Papillion Times over the weekend, Sherwin's mother, Cecilia Sherwin, said she and her family "refuse to believe the news" that their family member is a murderer.

Cecilia said that after the shooting, a young boy yelled for help and told her a man dressed in black had shot his family. Cecilia said Sherwin was last seen wearing green.

She also said the family traveled to the campground legally with a gun, the outlet reports.

"We think [Anthony] might have sensed trouble and grabbed the gun for safety," Cecilia said, per the outlet. "We refuse to believe the news. We are deeply saddened as he had so much to live for and gave us no indication that anything was wrong."

Cecilia added her family has "cooperated fully with the police and investigative team from the get-go."

During the initial investigation, Cecilia told authorities Sherwin was missing and she was concerned he might have also fallen victim to the shooter, according to the outlet.

Cecilia reportedly said her son "gave us no warning that he was planning anything of this sort."

"Anthony was not capable of this sort of violence," she added, according to the outlet.

Mortvedt tells PEOPLE they are still awaiting autopsy reports. More information on the triple-murder-suicide is expected to be available by week's end.

As of Wednesday afternoon, a GoFundMe campaign launched on behalf of Arlo has raised over $230,000.