The FBI has been called in to assist local authorities in rural Wisconsin after four young friends from Minnesota were found murdered and “randomly” left in an abandoned SUV in a cornfield.

Authorities in Dunn County, Wisconsin, where the quadruple homicide victims were discovered Sunday, have made no secret that they have been left baffled by the crime.

“It’s highly unusual for this area. Obviously we’ve had homicides in the last several years, but something of this magnitude—[I’m] working on my 33rd year working for Dunn County, and this is a first,” Dunn County Sheriff Kevin Bygd said at a news conference late Tuesday.

Perhaps most perplexing, the victims have been identified as longtime friends from the Twin Cities area—none of whom apparently had any connection to the area of Wisconsin where they were found.

“We can’t find any connection to this area other than possibly randomly driving out of the Twin Cities,” Bygd said.

The victims—30-year-old Nitosha Lee Flug-Presley, 30-year-old Jasmine Christine Sturm, 26-year-old Matthew Isiah Pettus, and 35-year-old Loyace Foreman III—all died of gunshot wounds, police said. They are believed to have been killed less than 24 hours before they were found by a farmer on Sunday morning.

Flug-Presley and Sturm are described as close friends; Pettus was reportedly Sturm’s half-brother, according to the Minneapolis Star Tribune, and Foreman was her boyfriend.

“This is a freak thing that never should have happened,” Zach Pettus, the brother of Matthew Pettus and half-brother of Sturm, told the Star Tribune. He said he couldn’t see any reason why anyone in the longtime group of friends would be targeted.

“We knew something was wrong when [Sturm] didn’t respond to our messages,” he was quoted saying. “We have an idea who did it, but I don’t know if I should say something now.”

Sturm and Foreman leave behind five children. Pettus had a 3-year-old daughter, and Flug-Presley was frequently pictured with two young children in her social-media posts.

The group had reportedly been together at a bar in St. Paul on Saturday night, located about 65 miles from where they would be found the next day. But they left with someone just hours before turning up dead, according to Flug-Presley’s dad.

Damone Presley Sr. told the Twin Cities Pioneer Press the group had left the bar together and gotten into somebody else’s vehicle.

But he said he didn’t know anything further than that.

“That is the mystery,” he was quoted telling the newspaper. “Hopefully, through the investigation we’ll get that answered,” he said, adding that the whole thing “just doesn’t make sense.”

Police have not yet named any suspects or revealed details about a possible motive, with Bygd going so far as to admit on Tuesday that even the motive is “a mystery” to investigators at this point.

Authorities have, however, said they do not believe the quadruple homicide had any connection to drug activity or organized crime.

The FBI Minneapolis field office is helping the sheriff’s office investigate the grisly killings, and it may turn into a joint investigation if it turns out the murders were committed in Minnesota, an FBI spokesman told the Pioneer Press.

For now, according to Bygd, investigators are starting to scrutinize everyone the group of friends knew.

“Everyone’s a suspect at this point,” he said.