The photos were posted on the Internet by a group at Fairchild Air Force Base that supports breast-feeding mothers. The group, Mom2Mom, said the photos were meant to promote World Breast-Feeding Week in August.
But state National Guard spokesman Capt. Keith Kosik said it's a violation of regulations to use the uniform to promote a civilian cause.
He said the Guard members likely will not be disciplined but that the incident would be used as an opportunity for education.
"If you look at the press coverage that's out there right now, it has been misconstrued as a battle against breast-feeding," Kosik told The Spokesman-Review newspaper on Thursday.
The military has no rules specifically regarding public breast-feeding while in uniform. The real issue is that servicemembers are not allowed to use the uniform to further a civilian cause, Kosik said.
The photos of the women quickly went viral and caused a stir around the world
"This was not what we were expecting," said Crystal Scott, the support group co-ordinator. "It just exploded."
The images have resulted in a flood of comments, many positive, some negative.
"I was so surprised at some of the comments," Scott said. "The negative ones, they're calling it a disgrace to the uniform."
Scott hopes the photos will encourage more women to breast-feed, including in public. The series of photos was taken by Brynja Sigurdardottir, a military spouse at Fairchild.
They feature two Fairchild service members and mothers, Terran Echegoyen-McCabe and Christina Luna, breast-feeding their babies while wearing their Washington Air National Guard uniforms.
Neither of the women was immediately available for comment.
Echegoyen-McCabe told The Spokesman-Review she did one interview before she was ordered to stop by her superior officers.
"They ordered me not to speak to the media," she told the newspaper.
Fairchild, located just outside Spokane, is home to the National Guard's 141st Air Refuelling Wing.