The three female Marine officers who made it through the grueling first exercise of Infantry Officer Course at the start of October were asked to leave after falling out of two hikes, Marine Corps officials said this week.

The second lieutenant and two captains were dropped from the 13-week course held aboard Marine Corps Base Quantico, Virginia, about two weeks after it began. The Marines got further in the course than any other women since IOC opened to female volunteers on an experimental basis in late 2012. Only one other female officer has gotten past the notoriously difficult combat endurance test that kicks off the course; she was forced to drop out about a week later due to stress fractures in her foot.

Capt. Maureen Krebs, a spokeswoman for Headquarters Marine Corps, said the two captains were both logistics officers, while the lieutenant was in training to become a combat engineer. Krebs said she could not release their names or ages due to privacy regulations governing the ongoing research.

The three officers were dropped from the course the week of Oct. 17, Krebs said, after failing to keep up on two long hikes while carrying an approach-march load of up to 120 pounds. The load represents a day's meals, clothing, supplies and assault gear for a 20-mile march into combat.

According to the infantry training and readiness manual, an infantry officer is expected to maintain a pace of 24.8 miles in eight hours, or approximately three miles per hour, carrying the approach-march load.

Marines who fall more than 100 meters behind a unit on one of these hikes at IOC are given a limited period of time to catch up, Krebs said. If they can't, she said they are put in a vehicle that will take them the remainder of the distance.

"The big thing in this is, they're expected to lead that tactical movement as an infantry officer," Krebs said.

The first hike, a seven-mile march to a range at Quantico took place during the second week of IOC. The second, a nine-mile hike to a range, happened the following week.

"Three men and three women failed to complete those two tactical movements," Krebs said.

They were all asked to leave on the same day, she said.

None of the three female officers will reattempt the course. Krebs said the lieutenant had already recycled once from a previous iteration of IOC, and the two captains declined the opportunity in order to return to their responsibilities in the fleet.

The course, which has 60 remaining participants, will conclude in December. At least one female officer is set to be part of the next iteration of IOC, which kicks off in January.

This course was the first since the Marine Corps changed its policies to allow experienced female lieutenants and captains from the fleet to volunteer for IOC alongside new lieutenants fresh from The Basic School. This change was designed to send more female volunteers through the course in order to expand the Corps' data collection as the Defense Department deadline to open all combat fields to women draws near.

By the start of 2016, all service chiefs must either concur with a recommendation to open all military specialties that remain closed to women or petition the defense secretary for specific exceptions based on research.