June 5, 2009 -- One Saturday night in March 2007, a college house party in San Jose, Calif., was under way. Members of the baseball team from nearby De Anza Community College were celebrating one ballplayer's birthday. Neighbors would later tell investigators that the house was a frequent crossroads for college kids and booze.

On the face of things, it was just another party. But it would end badly in a crowded room, and a frantic trip to the hospital.

In an interview with ABC News' "20/20," partygoers April Grolle, Lauren Chief Elk and Lauren Bryeans alleged that, at the end of the night, they witnessed some members of the baseball team gang rape a 17-year-old girl.

None of the three women, friends from the De Anza soccer team, knew the alleged victim, who seemed to be extremely intoxicated, they said. At some point, the 17-year-old found her way to a room with a group of ballplayers, they told "20/20."

According to Bryeans, 20 at the time, two ballplayers engaged in sexual activity with the high school student as she lay flat on her back on a mattress, apparently unconscious. Chief Elk said her pants appeared to have been "ripped off" and the mattress was surrounded by "about 10 pairs of legs." The 17-year-old had vomit on her body that later would be determined not to be her own.

Several investigations were launched to find out what happened in that room. Grolle, Chief Elk and Bryeans say they know.

Watch the full story tonight on "20/20."

The three women rushed the teen to an emergency room because they suspected alcohol poisoning and feared for her life, they said. Authorities were quickly called in to investigate a possible gang rape.

"We did hundreds, if not thousands, of hours of investigation," said Sheriff Laurie Smith of Santa Clara County, who headed up the case.

Click HERE to read Part 1 of the story.

Although authorities had the three eyewitness accounts, a major roadblock in the investigation would quickly surface. The teen had no memory of the incident.

Alleged Gang Rape: 'Impossible to File Charges'

"The investigation was frustrating to me," Smith said. "They were part of a baseball team from a college. ... I think there probably was a code of silence. They hung together and they didn't provide us with information that we needed."

The ballplayers did talk to their coach. Those who admitted that they were in the room were suspended from the team for violating the school's code of conduct. The sheriff's office concluded that the assault likely happened. The cased moved on to the Santa Clara District Attorney's office.

Then, in May 2007, the prosecutor's office made a decision that stunned Smith and others throughout the community. District Attorney Dolores Carr decided against filing criminal charges in the case.

"The way the case stands now, it is impossible to file charges and hope to get a conviction in this case," Carr said at the time.

Smith said, "The D.A. called me at home ... it was devastating."

Grolle, Chief Elk and Bryeans said they were crushed for the alleged victim. "There were three people who watched something happen," Chief Elk said. "And like, if that is not enough, then you know, why is rape even a crime?"

Local outrage ensued. The prosecutor drew heated criticism. Carr praised the three witnesses for rescuing a young girl from what she called a "horrible situation." But she insisted that what they saw that night wasn't enough to prosecute. The biggest problem for a jury, she said, would be that "the girl did not remember" what happened to her.

In the face of continued criticism, Carr called in State Attorney General Jerry Brown after two weeks to judge the evidence. The attorney general's investigation dragged on for nearly a year, offering a glimmer of hope for those who wanted a trial.

The baseball players, who said the girl had consented to have sex, had little public support. But defense lawyer John Cahners, who represented one of the ballplayers, said there is a side to the story the public never heard.

Alleged Gang Rape: No Woman Left Behind

"It was apparent to a lot of the witnesses that she wanted sex that night," Cahners said.

He cited a purported provocative dance between the girl and one of the ballplayers. "They call it, in a nice way, a lap dance; it was very, very sexual," the lawyer said. "If she's asking people to have sex with her, and if people are having sex with her, there's no ... that's not illegal. That's consensual. Now, if she's asking more than one, then that's up to her."

The alleged rape victim has said that she never consented to sex with anyone that night.

Brown decided last year not to file charges, citing "insufficient evidence for prosecution," questionable accounts from partygoers who had consumed "large amounts of alcohol" and, in general, "wildly conflicting statements."

Believing she was robbed of her day in criminal court, the alleged victim has now filed a civil suit for unspecified damages against nine of the De Anza ballplayers, accusing them of, among other things, rape and/or failure to stop an assault.

As for the De Anza baseball players, they declined repeated requests by "20/20" for an interview.

But one of the ballplayers' mothers, Jenna Skinner, sent an e-mail message.

"Talk to your kids about what happened here," wrote Skinner, whose son Chris is a defendant. "Don't think something like this can't happen to you. When you drink too much, you are not able to control what goes on around you. And you could end up in a nightmare like this, even if you don't do anything wrong."

As for Grolle, Chief Elk and Bryeans, they make appearances on college campuses, speaking for a campaign that they inspired called No Woman Left Behind. They tell their tale to auditoriums filled with college students.

"From that moment until now," said Chief Elk, who, like Grolle, was 19 at the time, "we will always know that we watched a rape happen."