A Massachusetts Air National Guardsman accused of posting secret intelligence reports online agreed to plead guilty on Monday in exchange for a sentence of up to about 16 years in prison and a pledge to comprehensively brief officials on what he leaked.

The airman, Jack Teixeira, withdrew his not-guilty plea during an appearance in Boston federal court and pleaded guilty to six counts of “willful retention and transmission of national defense information” under the Espionage Act, according to court documents.

The judge in the case, Indira Talwani, scheduled a hearing in September to determine whether she would sign off on the deal, which includes a sentencing range of 11 to over 16 years. It would be unusual for a judge to make major alterations to a deal that required approval from top American intelligence and law enforcement officials.

As part of the agreement, the Justice Department agreed not to charge Airman Teixeira, 22, with any additional violations of the Espionage Act, which, when combined with the other charges, could have resulted in a sentence of up to 60 years in prison had he been convicted.

Airman Teixeira, looking a bit younger than his age but stouter than in previous appearances, was led into a packed courtroom in an orange jumpsuit.

In the audience was Airman Teixeira’s mother, father and stepfather, members of the news media and a throng of curious court workers who filed in and out as the hourlong hearing wore on.

Prosecutors began by ticking off a list of Airman Teixeira’s reported misdeeds: how he made sure no one was around when printed out classified documents and posted them to the social media platform Discord; how he mistakenly believed he could maintain anonymity and “plausible deniability” online; how he deleted chats, killed a Discord channel and chucked his smashed iPad into a dumpster as police closed in.

When Judge Talwani asked if Airman Teixeira disagreed with any of the facts as stated, he responded simply, “No, your honor.”

At the end of the hearing, Judge Talwani asked the pivotal question. “Are you, in fact, guilty of the counts charged?”

“Yes, your honor,” he responded.

As he was handcuffed, Airman Teixeira turned and gave a tight, tense smile to his family before being led back into federal confinement.

Airman Teixeira has been in custody since being arrested at his mother’s house in North Dighton, Mass., last spring. He was responsible for one of the most far-reaching leaks of sensitive information in years — a huge embarrassment that revealed how even a low-level service member could retrieve and disclose defense secrets for months without being stopped.

Prosecutors said they found no evidence of espionage, and concluded that Airman Teixeira had posted secrets to a chat group on the social media platform Discord to impress people he met online with insider information, particularly details of the war in Ukraine.

“I’m not going to speculate on exactly what was his motivation,” said Joshua S. Levy, the acting U.S. attorney in Massachusetts, speaking after the hearing. “I think that’s something only he truly knows. We followed the leads where they went.”

A senior federal law enforcement official, speaking on the condition of anonymity to discuss the investigation publicly, said that the government would not have approved a reduced sentence if it had discovered Airman Teixeira had more nefarious motives.

Michael K. Bachrach, Airman Teixeira’s lawyer, said he would push hard for a sentence at the lower end of the range and cited his client’s youth and immaturity as an explanation for his reckless behavior.

“He is very much a kid,” Mr. Bachrach told reporters outside the courthouse. “We will be able to establish why his youth played a substantial role.”

Airman Teixeira feels remorse, he added.

His family blamed the Air Force for creating an environment that allowed him to retrieve and post secrets, which they called “a lackadaisical work atmosphere” with a “lack of adequate training and oversight,” in a statement.

Airman Teixeira, who worked at an intelligence unit at an air base on Cape Cod, took the material off computers after conducting unauthorized searches of databases, even after a superior warned him to stop, according to the original indictment.

Among the secrets disclosed was information on the provision and delivery of military equipment to Ukraine and a highly sensitive report on Russian and Ukrainian troop movements. Officials said the revelations about the troop movements might have compromised how American intelligence gathered the information and from whom.

He also shared a report on the hacking of an unnamed American company’s accounts by “a foreign adversary” and details of an unspecified foreign plot to target U.S. troops abroad that described “where and how” an assault might take place, the indictment said.

A New York Times investigation of more than 9,500 of his messages painted a portrait of a young man who was fixated on weapons, mass shootings and shadowy conspiracy theories.