A lawsuit filed this week claims to have uncovered hundreds of previously unknown cases of sexual abuse in the Boy Scouts, saying that the organization and other defendants engaged in “reckless misconduct” in failing to protect its young participants.

The lawsuit names one plaintiff, identified as S.D., who alleges he was assaulted hundreds of times, beginning when he was 12 or 13, by a Scout leader in Pennsylvania over about four years in the 1970s.

The complaint names the Boy Scouts, the Penn Mountains Council and S.D.’s alleged abuser as defendants and was filed in Philadelphia’s Court of Common Pleas.

The litigation stems from an attempt to unearth previously unreported cases of child sexual abuse in one of the country’s most prominent youth organizations, spearheaded by Abused in Scouting, a group of law firms that collaborate on bringing such cases to light.

In a statement to The Washington Post, the Boy Scouts said it had made about 120 reports to law enforcement based on information provided by Abused in Scouting.

“We care deeply about all victims of abuse and sincerely apologize to anyone who was harmed during their time in Scouting,” the Boy Scouts said.

“The BSA has taken significant steps over many years to ensure that we respond aggressively and effectively to reports of sexual abuse,” the statement continued. “We recognize, however, that there were instances in our organization’s history when cases were not addressed or handled in a manner consistent with our commitment to protect Scouts, the values of our organization, and the procedures we have in place today.”

The organization pointed to the use of screening efforts and background checks to prevent abusers from joining its ranks. The group also provides “youth protection education” for members and bars one-on-one interactions between adults and children. The Boy Scouts has a help line with which participants can report abuse.

For decades, the Boy Scouts organization has kept detailed files on “ineligible volunteers” that documented thousands of people considered to pose a risk of abusing children. In the past decade, a large tranche of the documents became public through lawsuits and investigative reporting.

But Abused in Scouting says it has discovered new accusations of abuse involving more than 350 people who do not appear in the ineligible-volunteer files, S.D.’s alleged abuser among them.

Most of the alleged assaults in the database of new cases viewed by The Post happened decades ago, though Abused in Scouting says it also has a 17-year-old client.

“This is the first time he’s ever come forward,” said Stewart Eisenberg, S.D.’s representative and one of the lead lawyers from Abused in Scouting. “He’s held it in for all those years.”

The suit alleges S.D., now 57, was the victim of an assistant scoutmaster who “actively groomed young boys under his charge for later sexual molestation,” subjecting S.D. to “hundreds of instances of fondling, hundreds of incidents of oral sexual assault and repeated attempts of anal penetration” at Camp Acahela, a Boy Scouts retreat in eastern Pennsylvania, as well as at his abuser’s home, the lawsuit says.

The lawsuit alleges that S.D.’s abuse would not have been possible had it not been for the negligence of the Boy Scouts. Requests for comment sent to phone numbers and emails associated with the alleged abuser were not immediately returned.