March 15, 2013 -- A critical eyewitness testified in the Steubenville, Ohio, rape trial this morning, saying he watched and recorded a high school football player accused of sexually assaulting a 16-year-old girl penetrate the alleged victim with his finger in the back seat of a car.
The witness, a 17-year-old boy, was in the car with the defendants Trent Mays, 17, and Ma'lik Richmond, 16, as they drove from one alcohol-fueled party to his home on the night of Aug. 11. All three boys played football together at Steubenville High School.
At the request of prosecutors, Judge Thomas Lipps granted the witness immunity from future prosecution for his testimony "in the furtherance of justice." The witness initially asserted his Fifth Amendment right not to incriminate himself on account of his recording an alleged sex act involving a minor.
The witness, who said he was in the back seat with Mays and the accuser, while Richmond was in the passenger seat, has not been charged with any crime.
ABC News has chosen not to identify the witness because he is a minor.
While in the car, "Trent [Mays] started fingering her, that's when I took out my phone and started filming," the witness testified, saying he was able to see the alleged victim's vagina.
The girl, he said, did not initiate contact with Mays and was only mumbling while in the car.
Prosecutors allege that Mays assaulted the alleged victim in the car and then again, along with Richmond, at the witness's home. There, some of the teens continued to drink alcohol and some, including the alleged victim, spent the night.
The girl, a 16-year-old from nearby West Virginia, "seemed drunk" and "wasn't really responding," the witness testified.
The witness said he deleted the video the next morning because "it was one of those moments you realize you did something wrong and stupid."
Walter Madison, the lawyer representing Richmond, was the first of the defendants' attorneys to cross-examine the witness.
Under cross, the witness said that of the three known photos taken that night, the only one to be made public was "a joke."
That photo shows the alleged victim being held by the wrists and ankles by two young men.
The witness's home was considered a crime scene, where samples of Mays' DNA were discovered on a blanket.
He said under cross-examination that he was "scared" when detectives questioned him and gave them information he hoped would keep himself "out of it."
Mays' attorneys are expected to further cross-examine the witness this afternoon.
Two other teenaged witnesses are also expected to testify today.
Today's testimony follows a dramatic day in court Thursday in which a forensic computer expert for the state introduced many of the often graphic messages sent from the alleged victim and the defendants in the hours after the party.
Most of the messages were between Mays and friends, in which the football player gives differing accounts of what took place at the party and how much sexual contact he had with the alleged victim.
In one instance, he claims to have had sex with the alleged victim, in other incidences he says the girl masturbated him and in others he says he digitally penetrated the girl.
In a series of texts between the alleged victim and friends, she pleads for information about what took place.
"Swear to God I don't remember doing anything with them," she texted a friend. "Wait I think I was drugged. I have no memory from after I left" the party, she texted one boy.
"I wasn't being a slut. They were taking advantage of me," she wrote.
Defense attorneys say a toxicology report performed a day later showed no signs of drugs.
After learning about some of the alleged incidents at the party, the girl emails Mays.
"Why the f--- would you let that happen. Seriously, you have no f---ing respect. People are telling me so much s--t, why the f--- would you take my clothes off in front of everyone. You shouldn't have let that happen," she wrote.
Richmond's lawyer declined to make an opening statement when the jury-less trial began Wednesday in juvenile court.
But he has told ABC News' "20/20" that what occurred that night was consensual.
Brian Duncan, a lawyer representing Mays, said simply: "Trent Mays did not rape the young lady in question."
Richmond, in an exclusive interview recently with "20/20" anchor Elizabeth Vargas, said, "I didn't rape anybody. I didn't witness a rape going on."
"And if I would have thought that somebody was being raped or anything like that, I would have stopped it."
If convicted, the teens could serve prison time until they turn 21.
The case drew further attention when some outside the small rustbelt town accused local officials of willfully protecting the football players, seen as hometown heroes.
For more on this story, watch "20/20" Friday, March 22 at 10 ET