WASHINGTON -- Cpl. Aaron C. Masa became fast friends with a fellow Marine during field training in North Carolina. But behind his buddy's back, Masa was sexually abusing his friend's 3-year-old stepdaughter. He also took sexually explicit photos of the girl and the Marine's infant daughter.

A military judge convicted Masa last year of sexual abuse of a child and production of child pornography, according to court records and other documents detailing the case. Under the terms of a pretrial agreement, he pleaded guilty and received 30 years in prison.

In total, incidents involving sexual assault in which the children of service members are victims occur hundreds of times each year, data the Defense Department provided exclusively to The Associated Press show. The abuse is committed most often by male enlisted troops, according to the data, followed by family members.

The figures offer greater insight into the sexual abuse of children committed by service members, a problem of uncertain scale due to a lack of transparency into the military's legal proceedings. With more than 1 million military dependents, the number of cases appears statistically small. But for a profession that prides itself on honor and discipline, any episodes of abuse cast a pall.

Those numbers fall well-short of offering a full picture.

The ages of the offenders and victims, the locations of the incidents and the branch of service that received the report of sexual abuse were omitted. The Defense Department said in a statement that "information that could unintentionally uniquely identify victims was withheld from release to eliminate possible 're-victimization' of the innocent."

It's also unclear how many of the incidents resulted in legal action. The cases represent substantiated occurrences of child sexual abuse reported to the Defense Department's Family Advocacy Program, which does not track judicial proceedings, the department said.

An AP investigation published in November found more inmates are in military prisons for child sex crimes than for any other offense. But the military's opaque justice system keeps the public from knowing the full extent of their crimes or how much time they spend behind bars.

Responding to AP's findings, three Democratic senators have urged Defense Secretary Ash Carter to lift what they called the military justice system's "cloak of secrecy" and make records from sex-crimes trials readily accessible.

The senators also raised another concern. Child sex-assault cases are not included in the Defense Department's annual report to Congress on sexual assaults, which focuses primarily on adult-on-adult incidents, they said. The senators -- Barbara Boxer of California, Kirsten Gillibrand of New York and Mazie Hirono of Hawaii -- told Carter in a Dec. 8 letter they are concerned the department may be underestimating how many sexual assaults are occurring in the military.

There were at least 1,584 substantiated cases of military dependents being sexually abused between fiscal years 2010 and 2014, according to the data. Enlisted service members sexually abused children in 840 cases. Family members of the victims accounted for the second largest category with 332 cases.

Most of the enlisted offenders were males whose ranks ranged between E-4 and E-6. In the Marine Corps and Army, for example, those troops are corporals, sergeants and staff sergeants. Officers were involved in 49 of the cases. The victims were overwhelmingly female.

Kathy Robertson, manager of the Family Advocacy Program, said in an emailed response to questions that the incident rates reflect the U.S. military's demographics. Most of the cases involve the E-4 and E-6 ranks because they are the largest number of active-duty personnel and the largest number of parents in the military, she said.

Duplications in the data indicate that as many as 160 additional cases of sexual abuse could have occurred during the 2010 to 2014 period, involving a child who was victimized multiple times or a repeat abuser. The figures also account only for cases involving military dependents, which are the only child victims the department tracks.

In Masa's case, military authorities first became aware of the alleged abuse in June 2014 after the 3-year-old told a neighbor that she did not like Masa because he touched her in certain places and "made it hurt," according to the Naval Criminal Investigative Service's investigation obtained through the Freedom of Information Act.

NCIS blacked out all the names in the report, including Masa's. The AP identified him by the dates and events in the document that matched records from Masa's court-martial.

Masa, 24, admitted during questioning to at least five instances of sexually assaulting her, the NCIS report said. Investigators found nude photos of both girls on his cellphone and later discovered a pair of girl's underwear during a search of his mother's home in Ohio.

The report depicted Masa as a loner with a troubled past. People interviewed by the Navy investigators described him as "oddball" who was picked on in high school in Marietta, Ohio. He graduated near the bottom of his class with a cumulative 1.782 GPA, according to his official transcript.

Masa watched a lot of sexually explicit Japanese animation known as hentai, the NCIS report said, and he had an intense interest in "furry porn," a genre of pornography in which animal characters with human arms and legs engage in sex.

In 2008, Masa was arrested after threatening to bring a gun to school and shoot three other students, according to the NCIS report, which included details of the incident. Police didn't find a gun on him, but he had a knife. He pleaded guilty to a disorderly conduct charge.

He enlisted in the Marine Corps in June 2010, a month after graduating from high school. The arrest wasn't serious enough to bar Masa from enlisting. A Marine Corps spokesman said he was found qualified after a thorough screening process that involved physical, mental and moral evaluations.

Stationed at Camp Lejeune, a sprawling installation on the North Carolina coast, Masa started a friendship with the Marine sergeant and his family in 2013, according to the 286-page NCIS report. They fished together and chopped wood for the sergeant's fire pit. Masa loaned the sergeant money to help the family through a financially difficult period and spent more and more time at their home, the report said. He babysat the girls and volunteered to give the older daughter a bath, the documents said.

Amid the good will, suspicions surfaced. The girls' mother suspected as early as March 2014 that the 3-year-old daughter was being molested because she complained of pain "down there," according to portions of medical records from a hospital near Camp Lejeune that are included in the NCIS report. The girl was sleeping by the door and having nightmares, the mother said.

Yet at the hospital, the girl was in high spirits, smiling, laughing and jumping on the bed, according to the report. She was diagnosed as having a urinary tract infection and antibiotics were prescribed. "Mom advised to follow up with law enforcement if she has concern about possible molestation," the report said, quoting the medical records.

But an investigation wouldn't be launched until a few months later, after the girl talked to the neighbor. While being questioned by investigators, Masa drew a diagram of the floor plan of the sergeant's home, using X's and O's to show where the abuse occurred.

Masa is serving his sentence at the U.S. Disciplinary Barracks at Fort Leavenworth, Kansas.