NEW YORK — As a Marine infantry sergeant, James LaPorta once led an intelligence team in Afghanistan. Now, as a private citizen, he is doggedly tracking the moves of an online group that has been secretly compiling and sharing nude photos of hundreds of women in the Marine Corps.

With top generals admitting before Congress that they are unsure how to protect members of the Marine Corps from nameless, faceless social-media predators, LaPorta is among an unlikely scattering of can-do young veterans who have decided to take that assignment upon themselves.

He and his comrades in online vigilance have been gathering intelligence and making counterstrikes. They are tracking the members of illicit groups, including Marines United, as they try to hide, and stripping away the anonymity that has allowed the group to thrive. They are also feeding information back to Marine Corps investigators.

“The Marines’ response is to be careful and slow, but the people they are after move very fast,’’ LaPorta said. “If you want to catch them, you have to move at their speed.’’

Indeed, Marine Corps leaders have resorted to traditional moves, commissioning a task force and mounting a meticulous investigation. But the commandant of the Marine Corps, General Robert B. Neller, acknowledged to the Senate Armed Services Committee on Tuesday that the nude-photo scandal was a cultural problem he was ill-prepared to address.

It is certainly not easy: Marines United, which consists of thousands of active-duty and veteran Marines, has hopped from Facebook page to Facebook page, changing its name each time it gets shut down, while still trading illicit photos and taunting federal investigators.

“The Marine Corps thought because they shut a Facebook page down, the group was dead,’’ said LaPorta, 30, who left the Marines in 2014. “We had to show them it was just metastasizing into other back rooms.’’

LaPorta and other veterans trying to fight groups like Marines United have been deluged with online harassment themselves. Other Marines have called them traitors and threatened them with violence, but they have pressed on in what they see as a battle for the future of the Corps.

“The Marine Corps can’t do this alone. The Internet is too huge,’’ LaPorta said.

“There is a disconnect between the upper echelon and a digital millennial generation,’’ said Thomas Brennan, 31, a former Marine sergeant who revealed the existence of Marines United this month.

A Marine spokesman said he could not comment on the ongoing investigation.

Earlier this week, Representative Jackie Speier, Democrat of California, announced that investigators had identified 700 active-duty Marines and 150 Marines in the Reserve who were involved in the Marines United group.

For several years, the Marine Corps has known of webpages where Marines shared racist and sexist memes, as well as photos of female Marines posted without their consent. Despite several public reports, the Corps has failed to crack down on the sites.

Active-duty and veteran Marines in a group called Just the Tip of the Spear have been posting illegal and offensive material on Facebook, Instagram, and other sites.

Like Marines United, they have posted nude photos of servicewomen without their permission. In some cases, they have also posted the women’s telephone numbers and other private information. When women complained, the site’s followers often harassed them more.