MARINE CORPS BASE QUANTICO, Va. — In a cramped office at the Marines' Quantico base outside Washington, about 20 investigators sit elbow to elbow, staring into their computers as images of naked men and women flash across the screens.

On the walls are white boards with statistics, crime lists and a montage of social media messages directed to the Naval Criminal Investigative Service.

The objective of this disturbing sleuth work: Rooting out the extent of a nude photo-sharing scandal that has rocked the Corps, embarrassed its leaders and spread to other military services. And the sheer scope of the job is daunting.

"If you do that eight to 10 hours a day, five days a week, you get pretty burned out," NCIS Director Andrew Traver said in an interview. New agents cycled in after the first month, he said, "just because of the burnout factor, especially the ones that are doing the image review."

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This is Task Force Purple Harbor. What began as a response to military members posting nude photos online has morphed into a growing criminal investigation that now includes 21 felony cases and more than 30 others referred to Marine commanders for possible administrative action. Five Marines have received administrative punishments so far, but no details have been provided.

For the investigators, men and women, it is a broad and grueling process. Agents from all four services and the Coast Guard have scoured close to 200 different websites. They've pulled more than 150,000 nude or semi-nude images. They've identified 20,000 with a possible military connection. More than half are of men.

The overwhelming majority are selfies or photos subjects posed for and then voluntarily shared, which is not illegal even under military code. That leaves just a small number of people who could potentially be prosecuted for crimes such as extortion and stealing or hacking into someone's computer hard drive.

More than a dozen military members — mostly women — have asked the task force for help. They want to know if any of their intimate photographs ended up on the largely private websites without their consent. In four cases, facial recognition software has helped identify victims. One woman confirmed an image was of her. The other three are still checking.

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Andrew Traver serves as director of the Naval Criminal Investigative Service (NCIS). Carolyn Kaster / AP

It's the kind of investigation that could go on forever. A simple word search of "uniformed military nude" got nearly 80 million hits, Traver told The Associated Press during the interview in his Quantico office. And the anonymity of the internet makes it difficult to identify either suspects and victims.

"People that are posting and reposting, generally are not posting under their real name and some people are posting under multiple identities," said Traver. "It's very difficult for us to find the origin, to find the person that started the whole chain."

In the computer room, agents stare into their computer screens for hours a day. Tucked between the keyboards are cups, scattered papers, Fig Newtons, a cereal box.

The investigators are reviewing seemingly endless images to ferret out those they can link to the military and that appear to have been posted without permission.

Related: 2 Marines Demoted, More Investigated In Nude Photo Probe

It's a moving target. Site hosts shut pages down and open new ones, often within a day or two. The websites are usually private and require an invitation, which is vetted by hosts demanding a "tax." The tax, or entry requirement, is usually a naked photo, said Curtis Evans, chief of the NCIS criminal operations division.

"As law enforcement, we can't do that, and they know it," Evans said.

He described some of the internet memes online that taunt the investigators. One is a gorilla wearing an NCIS hat with a sign saying "add me." Others mimic Facebook pages, saying they are open to "Friends and NCIS" or "Only Media and NCIS."

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When an investigator does secure an invitation into a closed website, Traver added, "we don't last long." When the hosts see that someone isn't posting photos or making comments, the person is kicked out.

Still, investigators have made headway.

They've opened 21 criminal cases. Sixteen suspects have been identified: nine active duty Marines, two Marine reservists, three Navy sailors, one Navy reservist and a civilian. Suspected crimes include extortion, stalking, threats and theft of photos.

NCIS' cyber experts are providing help sorting through the massive reams of data. They've developed new tools and software to analyze photos more quickly. All the data on a website can be stripped and each photo reviewed to determine if it includes a military uniform, a male or female, signs of consent.

A process that once took 40 seconds now takes only five to 10.

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Perpetrators are getting quicker, too. The task force has seen cases when someone has posted a photo online of a military member in uniform, snapped perhaps in the chow hall or on base, and then asked for naked images of that person. Traver said it sometimes takes only an hour-and-a-half for someone else to post a naked or semi-nude photo in response.

After two months, Traver said, the investigation has slowed. They're now working more on individual cases than on the initial internet photo search.

"Now that we're sensitized to this, this will become another aspect of that type of work that we do in cyberspace," Traver said. "It's another realm that we hadn't really seen before."