A female Marine lieutenant quietly crossed one of the last remaining hurdles for women in the service Monday, celebrating graduation of the famously difficult infantry officer course in a private ceremony reserved for family members.

The officer, who asked not to be identified, is the first woman in the Marine Corps to earn the 0302 infantry officer military occupational specialty. She will soon be sent to her new unit, which is within 1st Marine Division, out of Camp Pendleton, California, officials with Marine Corps Training and Education Command said Monday morning.

"I am proud of this officer and those in her class‎ who have earned the infantry officer MOS," Marine Corps Commandant Gen. Robert Neller said in a statement.

Some 32 female officers have previously attempted the 13-week infantry officer course at Quantico, Virginia since the course first opened to women on an experimental basis in 2012. The vast majority of applicants did not make it past the grueling combat endurance test that kicks off the course. The test runs more than 14 hours and is designed to test Marine officers mentally as well as physically, forcing them to hike many miles with 80-pound packs and complete an obstacle course in which they must climb a 20-foot rope multiple times.

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The Marine Corps has at times faced pressure to reconsider the difficulty level of IOC as it proved a much more formidable challenge than the enlisted Infantry Training Battalion course, which has graduated well over 200 female Marines to date, some of whom have gone on to take positions in operational infantry units.

But leaders remained adamant that they wouldn't change standards to make a quota.

"The important thing is that it's open for anybody who qualifies," then-Navy Secretary Ray Mabus told reporters in January. "It's just that you have a chance to make it through."

In all, 27 female officers attempted IOC on an experimental basis from 2012 to 2015, while infantry jobs still remained closed to women. Two more tried to pass the course during the same time period as a prerequisite for a ground intelligence MOS.

During that same time period, 978 male officers attempted IOC and 692 graduated, TECOM spokesman Capt. Joshua Pena previously told Military.com.

Since the Marine Corps opened the infantry and all other previously male-only jobs in 2016 in compliance with a Pentagon-wide mandate, four female officers have attempted the course, including the lieutenant who graduated today, Pena said.

Though the recent IOC class, which kicked off in July, has proceeded in privacy, with little exposure to the public eye, there were indicators early on that Marine officials sensed history was about to be made.

"We have a female officer right now in infantry officers course, and she's part of the way through, doing very well," Assistant Commandant of the Marine Corps Gen. Glenn Walters told reporters at the Pentagon in early August. "These are successes that never seem to get out in the press."

IOC instructors and TECOM leadership have declined to further discuss IOC and the lieutenant's history-making accomplishment.

In all, 131 Marines started the course 13 weeks ago, and 88 graduated today, officials said, giving the course a 67 percent graduation rate.

"Marines expect and rightfully deserve competent and capable leaders, and these IOC graduates met every training requirement as they prepare for the next challenge of leading infantry Marines; ultimately, in combat," Neller said.

-- Hope Hodge Seck can be reached at hope.seck@military.com. Follow her on Twitter at @HopeSeck.

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