The parents of the assailant who killed six people at a Nashville Christian school in March will transfer legal ownership of the writings their child left behind to the families of roughly 100 students, providing unexpected support to those families’ efforts to prevent the release of the documents.

The surprise decision, outlined in a Tennessee courtroom on Thursday, could prove crucial in an increasingly fierce legal battle over whether the writings should be made public to shed light on the shooter’s motivations or kept private to shield the victims from further pain.

The lawyer for the shooter’s parents, David Raybin, did not say in court how or why they had come to the decision. But speaking to reporters, he acknowledged that it strengthened the argument that the families should be allowed to participate in a lawsuit aimed at forcing the release of the writings as a matter of public record.

The parents of students at the Covenant School, along with the school itself and the adjoining church, have said the writings should never be released, citing fears of inspiring another mass shooting and further traumatizing their children. An opposing coalition that includes news outlets and a gun rights group has argued that the writings, which are now in the police’s possession, are part of the public record and crucial to understanding why the violence occurred.

Brent Leatherwood, a Covenant School parent who has been present in court over the last few weeks, said Thursday’s developments made it “an extraordinary day by any definition.”

“From the outset of this case,” Mr. Leatherwood said in a statement, “we have told our attorneys to leave no stone unturned in our pursuit of the ultimate objective we have, which is to prevent any of these writings or any material that might inspire a similar destructive event from ever seeing the light of day.” He added, “No one — either in the legal community or outside of it — should doubt our resolve.”

The twist on Thursday came days after a judge, Chancellor I’Ashea L. Myles, ruled that the parents could intervene in a lawsuit against the City of Nashville over the publication of the shooter’s writings. The groups seeking the release of the documents have appealed this decision and questioned whether the Covenant parents have the legal protections granted to the victims of a crime.

During Thursday’s hearing, Mr. Raybin announced that the shooter’s parents, Ronald and Norma Hale, had agreed to hand over legal ownership of the documents to the parents of the Covenant School students on behalf of their children. The formal paperwork will be completed in the coming days and filed with the court.

The police shot and killed the assailant within minutes of entering the school, but are continuing to investigate what led to the March 27 shooting, in which three 9-year-olds and three adults were killed. Because the Hales’ child did not leave behind a will, Mr. Raybin said, they, as the most immediate surviving relatives, had become the legal owners of the documents that the police took as evidence. (The couple, Mr. Raybin said, also owned the vehicle and the home where the writings were found.)

Chancellor Myles did not immediately issue a decision on whether the broader debate over the release of the documents would proceed in court. The decision, she observed, was likely to face an appeal.

If the Covenant School parents succeed in their case to keep the writings private, it is unclear what they would do with the documents after the police investigation concludes. In court documents, they have signaled a willingness to release some police reports and summaries, as well as what they described as “a police summary of the shooter’s motivations (or lack thereof).”