Groton — A Navy sailor will serve 22 years in confinement for sexually assaulting minors and possessing and producing child pornography.

Hospital Corpsman First Class Tristan J. Thomas, 31, was charged with 26 specifications, or counts, in violation of seven articles of the military's criminal code. He pleaded guilty to 22 of them.

The full sentence is 70 years, suspended after 22 years served, in accordance with a pretrial agreement.

Most of the charges stemmed from a two-year sexual relationship Thomas said he had with a teenager that began when she was 14, and a three-year relationship with another teen who was 15. There were a total of four victims between the ages of 13 and 16 in the case, which was tried before a military judge in a two-day court-martial at the Naval Submarine Base.

One of the victims, who is now 17, attended the proceedings with her father, and they both addressed the court before Thomas was sentenced Tuesday. She told the judge that when she met Thomas in February 2011, she was lonely because of issues she was having at home and at school, and he "used" her for two years. She said she attempted suicide, in part because of the toll the sexual relationship took on her.

"I want him to know how badly he hurt me," she said. "… I want him to know he took my childhood from me."

Thomas, who wore his service dress blue uniform from the Groton-based USS Springfield, chose to make an unsworn statement to the court. He began by standing to face the victim and apologizing to her, to her family and to the other victims. He spoke about his own troubles growing up and about becoming a victim of sexual assault at the age of 14 and again at 20, but he said what happened to him did not excuse his behavior.

Thomas has a 5-year-old son. Being away from him, Thomas said, "is the cross I will have to bear for causing pain to so many other children and grief for their families."

Two people he served with on the Springfield and his father, mother and sister also spoke on his behalf.

Lt. Cmdr. William H. Weiland, the senior trial counsel for the Region Legal Service Office Mid-Atlantic, asked on behalf of the government that Thomas be sentenced to life in prison with the possibility of parole and dishonorably discharged from the Navy. Weiland said it was "hard to put into words the depravity" of Thomas' actions, since he preyed on the naiveté and insecurity of young girls.

Lt. James M. Belforti, for the defense, said Thomas "has done monstrous things, but he's not a monster." He asked for a 13-year sentence.

"He robbed the childhood of others, he shouldn't be able to enjoy the childhood of his son," Belforti said, adding that the length of the sentence would give Thomas "time to heal." A psychological report found Thomas suffers from post-traumatic stress disorder, Belforti said.

In addition to confinement, Thomas was sentenced to a reduction in rate to the lowest pay grade and a dishonorable discharge for violating the articles of the Uniform Code of Military Justice. He was also charged for possessing and producing child pornography under the general article. His military pay will automatically be forfeited, and he will be required to register as a sex offender.

Unlike in civilian courts, after Thomas entered his guilty plea Monday, the military judge, Marine Corps Lt. Col. Charles C. Hale, extensively questioned him to make sure he understood exactly what he was admitting to.

Over the course of eight hours, Hale meticulously explained each element of every charge and repeatedly asked Thomas to explain how his actions violated the articles, how he did so willingly and was not coerced, how he knew the victims were underage and how he discredited the service.

Thomas said he had a good understanding of anatomy and child development because of his medical training and the fact that he is a father. He explained how he downloaded and kept child pornography on his computer and external hard drives, and neither deleted them nor reported the sender to law enforcement. He said he sent explicit images of himself to some of the victims and recorded sexual acts with one of the teenagers on his cellphone. And he described the sexual acts with the victims.

Hale found Thomas guilty on all of the counts he pleaded to.

In his 11 years in the Navy, Thomas, of Cincinnati, said he has served on two carriers and a submarine, as well as spent 18 months in Afghanistan mostly working with the Army. He reported to the Springfield in May 2011 as the sole medical provider on the boat and qualified as a submariner while on board.

Thomas faced a maximum sentence of life without parole, total forfeiture of pay and allowances, a reduction in rate and a dishonorable discharge. He has been held at the Wyatt Detention Facility in Central Falls, R.I., since January.

j.mcdermott@theday.com