The Pentagon is investigation the image-sharing - and in the past 24 hours, both CBS News and Business Insider have reported that members of other branches of the USA military were involved in similar activities, and that online bulletin boards were being used to either share or solicit nude or explicit photos and videos of women in the military.
As a criminal investigation into the sites begin, with hundreds of Marines implicated, Senior Master Sgt. Ronald Green, the Marine's top enlisted man, called Marines United the "tip of the spear" while speaking to Congress.
"We will not excuse or tolerate such behavior if we are to uphold our values and maintain our ability to defeat the enemy on the battlefield", Mattis said. Marine Corps commandant General Robert B. Neller said on Sunday: "For anyone to target one of our marines, online or otherwise, in an inappropriate manner, is distasteful and shows an absence of respect".
The story was first reported by The War Horse, a news group run by Marine veteran Thomas Brennan. The group of female Marines, which has its own Facebook page, formed in response to the controversy.
The US Naval Criminal Investigative Service (NCIS) has launched a probe into the allegations after a report by a US-based non-profit news organisation uncovered a "secret" Facebook group - which has several Marines as members - that posted naked pictures of US service women. Business Insider reported that a link to a DropBox folder called "Girls of MU" (Marines United) was being passed around.
Neller said all Marines, male or female, need to come forward to report abuse and pledged the service would not retaliate against those who do.
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"They're going to look at what's going on, while developing plans for corrective actions and recommendations to policies, procedures, education and training of Marines that will prevent this in the future and the culture - I'd say subculture - that may have given rise to this", he said.
She was accompanied by Marisa Woytek, an active-duty Marine who said in a written statement that while she was "fully clothed and appropriately dressed" in the photos posted of her, those images drew comments suggestive of sexual violence. She's also a captain in the Marine Corps Reserves. Some suggested in the 2,500 photo comments that the servicemen perform sexual acts with the women and film it for the other members. It will also, he said, address underlying cultural issues that may have created a permissive environment for bad behavior online.
The photos often include the names of the women pictured or where they're stationed.
Another victim, former Marine Kally Wayne, told ABC News for a report published Wednesday that her ex-boyfriend, also a Marine, had posted a sex tape of the two of them together to the Facebook group.
"From the marines who have reached out to us since the Marines United story emerged, we believe these problems are pervasive", said Colonel Don Christensen, former chief prosecutor of the U.S. air force and president of Protect Our Defenders.
"I felt like my privacy had been taken away from me", Wayne said.