Michigan authorities are moving forward with a broad investigation into Tuesday’s school shooting, including looking into whether the suspect’s parents had an accomplice and what actions school officials took after they were warned about the teenager’s allegedly disturbing behavior.

Appearing on video Saturday morning, the parents, James and Jennifer Crumbley, pleaded not guilty to four counts each of involuntary manslaughter, charges that allege that they indirectly contributed to the deaths of four teens Tuesday at Oxford High School, about 40 miles north of Detroit.

Oakland County Prosecutor Karen McDonald has alleged culpability in detailing the charges, saying the parents bought the suspect the gun used in the shooting and failed to act when school staff members warned them about his disturbing behavior.

“These two individuals could have stopped it,” McDonald said at the couple’s arraignment Saturday morning.

Shannon Smith, one of two attorneys representing the Crumbleys, said in the court session: “Our clients are going to fight these charges. Our clients are just as devastated as everybody else.”

The defense argued Saturday morning that the suspect was not given access to a gun.

“That gun was actually locked,” the Crumbleys’ co-counsel, Mariell Lehman, said in court. “So when the prosecution is stating that this child had free access to a gun, that is just absolutely not true.”

Oakland County Sheriff Michael Bouchard said at a news conference Saturday that the Crumbleys appeared to be reluctant to surrender, even though their legal team had offered reassurances that they would.

Bouchard said law enforcement began to look for the Crumbleys with warrants in hand. Officials were informed that the couple were not responding to texts or phone calls from one of their attorneys, he said.

They were found in what was described as an art studio in a building near downtown Detroit late Friday after someone spotted their vehicle parked outside and contacted authorities, Bouchard said.

The caller might be eligible for as much as $20,000 that was offered for information leading to the Crumbleys’ apprehension, U.S. Marshal Owen Cypher said at the news conference.

Bouchard said: “When the tip came in, the person was apparently outside smoking, and they pretty much ran away. Where they were and how they were seems to support the position they were hiding and they weren’t looking for surrendering at that point.”

He credited Detroit Police Chief James E. White for the crucial role his department, which scrambled quickly and set up a perimeter in an area near the Canadian border, played in capturing the two.

As the duo were alleged to be on the run, their attorneys insisted that they would be at a postponed arraignment Saturday morning, and they were after they were apprehended.

“Were they actually going to do it?” Bouchard said. “I don’t know. But given that they were hiding in a warehouse in Detroit, that certainly raises my eyebrows.”

Officials are investigating whether someone at the Detroit location helped the couple, and it will ultimately be up to prosecutors to weigh charges, Bouchard said.

“We believe they were assisted,” he said.

The Crumbleys and their son are all being held in isolation at the Oakland County jail, Bouchard said.

Four students were killed in the attack: Hana St. Juliana, 14; Madisyn Baldwin, 17; Tate Myre, 16; and Justin Shilling, 17. Seven other people, including a teacher, were wounded.

Authorities said a teacher raised the issue that the suspect appeared to have been searching for ammunition on his cellphone a day before the attack.

And on the day of the shooting, the Crumbleys were summoned to speak to administrators after the teenager was alleged to have produced disturbing images, including a gun, ammunition and the words “blood everywhere,” McDonald said.

The boy was not pulled out of school, nor was he barred from campus, she indicated; the attack took place after that meeting.

Bouchard suggested that the school should have notified authorities when the Crumbleys were summoned.

“At that point we would have loved to have been looped in,” he said.

He expressed concern for students and law enforcement officers who witnessed the violence and its immediate aftermath.

“There were some 18 unexpended rounds,” Bouchard said. “Could have been 18 more kids.”

Tim Throne, the superintendent of Oakland Community Schools, said in a letter to the district’s community Saturday that he has asked for a “third-party review” of administrators’ actions before the shooting.

But he appeared to defend their decision to keep the suspect, identified by authorities as Ethan Crumbley, 15, on campus.

“Given the fact that the child had no prior disciplinary infractions, the decision was made he would be returned to the classroom rather than sent home to an empty house,” Throne said.

Bouchard did not rule out including school officials in the investigation, but he reiterated that any charges were the province of prosecutors.

“Everything that happened, from preceding to that point to after till we stand here today, will be under investigation,” Bouchard said. “Every tidbit we learn will be handed over to our prosecutor for … follow-on charges, if applicable.”