Federal agents on Thursday arrested Jack Teixeria, a 21-year-old intelligence operative with the Massachusetts Air National Guard, who is believed to be the prime suspect behind the Pentagon-documents leaks.

“This is a law enforcement matter,” Brigadier General Patrick S. Ryder said during a Pentagon news conference on Thursday, declining to answer specific questions. “There is an ongoing investigation.” He added: “This was a deliberate criminal act. `

National-security leaks fall into two categories according to former Deputy Attorney General Rod Rosenstein: “easy and hard.”

“The hard ones are where you have to find the source. The easy ones are where you know who leaked it. This seems, right now, to be an easy one.”