For the man prosecuting two teen girls for allegedly assaulting an autistic schoolmate, the highly publicized Southern Maryland case is fairly routine in one regard: cellphone videos and text messages have provided plenty of evidence.

St. Mary’s County assistant state’s attorney John Pleisse said Friday that what investigators found on the cellphones of the girls, 15 and 17, is making his job far easier. On Thursday, the younger of the two girls, a student at Chopticon High School, was sentenced to up to six years in a state juvenile detention center for her role in tormenting the 16-year-old autistic boy.

The 15-year-old’s cellphone had been used to record videos of the attacks, including a knife allegedly being held to the boy’s neck by 17-year-old Lauren A. Bush, and the boy’s repeated falls through an icy pond after being persuaded to fetch a stray basketball. And Pleisse said both girls’ cellphones revealed numerous damning text messages.

After they learned they were under investigation in early March, they showed little remorse in their texts to one another, Pleisse said. “They were saying things like, ‘The only thing we did wrong was we were stupid, and we videotaped it, and we won’t do that again,’ ” the prosecutor said.

In late May, a St. Mary’s County judge will decide whether to transfer Bush’s case from an adult court to a juvenile court. Bush, who has been charged as an adult with first-degree assault, false imprisonment and child-pornography solicitation, is currently in a state juvenile facility undergoing psychological examinations.

Pleisse said the constant use of cellphones by teens is a prosecutor’s dream.

“I am having more cases where I have more and better evidence than I could ever imagine having,” Pleisse said. “I had one kid and he and a girl were exchanging [salacious] pictures of each other, and he sent a picture of himself — not his face — by mistake to the girl’s mother. That was a big oops. The next big oops? The girl’s mother showed the picture to her daughter and the daughter said, ‘Oh, that’s so-and-so.’ It’s almost laughable.”

During Thursday’s sentencing, the 15-year-old girl and her parents pleaded with St. Mary’s County Judge Michael Stamm that she be sent to a group home, not a juvenile detention center. The family had already gotten the girl accepted into a group home operated by San Mar, a privately run Maryland’s children’s center, where she’d have more freedom.

The judge refused, saying he considered the girl dangerous to the community.

But Bruce Anderson, San Mar’s chief executive, disagreed. In an interview, he said his deputy interviewed the girl and her family and spoke with others in the community.

“We’ve turned away many kids who we have not felt that they were safe, but we did not feel this girl was presenting a threat of danger to anyone at this point,” he said. “I had a feeling as if there was a lot of pressure on the judge to make a statement.”