SAN FRANCISCO — A prestigious all-male a cappella singing group from Yale, a late-night rendition of “The Star-Spangled Banner,” and a mysterious woman who may have vanished after saving one of the Yale men during a New Year’s Eve melee are all elements of the novel-like tale a judge is hearing here as she weighs whether two local men should stand trial on felony assault charges.

Richard Aicardi and Brian Dwyer, both 20 and recent graduates of Sacred Heart Preparatory High School, are charged with attacking two members of Yale’s Baker’s Dozen chorus in the wee hours of New Year’s Day 2007 following a party at a San Francisco home.

Judge Kathleen Kelly yesterday heard a police investigator recount a warning a witness allegedly gave the Yale singers after they were involved in a heated argument with Sacred Heart graduates at the party.

“You don’t want to mess with these guys. They will hurt you. They don’t care how much money you have or where you are from,” Inspector Richard Martin said a fellow partygoer, Jeremiah Kelleher, recalled saying.

Kelleher said he asked the local men to back down, to which Mr. Aicardi replied that he was “not worried about it because his friends are rolling 20 deep,” the police inspector said. The reference seemed to be to a van filled with Sacred Heart grads and other men that was en route to the scene.

As guests filtered out of the party, several fights broke out between groups Inspector Martin described in true town-versus-gown style as “the Yale boys and the city kids.”

The Yale students complained that the initial police investigation was suspiciously slow and lax, perhaps owing to the connections of Mr. Aicardi’s mother, Eileen, a prominent pediatrician who caters to many of the Bay Area’s wealthiest families. The probe seemed to pick up speed after one Yale student whose jaw was broken in the fracas, Sharyar Aziz Jr., appeared on television with his jaw wired shut.

The case also became a hot potato in gay-friendly San Francisco because some of the local men allegedly hurled anti-gay slurs after the singers performed “The Star-Spangled Banner” for partygoers. Yesterday and Monday, a total of three Yale students testified about the alleged assaults. According to lawyers, one Yale student, Bryan Bibler, said he saw Mr. Aicardi, clad in a Santa Claus hat, punch Mr. Aziz. However, the assault on Mr. Aziz is not one specifically charged against either defendant.

Mr. Aicardi’s lawyer, James Collins, accused the Yale men of “exaggerating” details of a fight in which his client acted in self-defense. The attorney noted that in earlier statements Mr. Bibler had said he did not know the identity of the man who punched Mr. Aziz. “The psychology department at Yale is really good if it can make your memory better after 18 months,” Mr. Collins said.

Under defense questioning, Inspector Martin conceded that police found Messrs. Aicardi and Dwyer to be cooperative at the scene and to show no signs they had just been in a fight, while some of the Yale men were inebriated and uncooperative.

Mr. Dwyer’s lawyer, Anthony Brass, challenged the credibility of the prosecution’s witnesses by pointing out that while two of the alleged victims said a woman laid on top of one Yale student to stop the local men from kicking him, Kelleher said he intervened to stop the assault and never saw the woman.

“I interviewed a lot of people and nobody saw this girl,” Inspector Martin said.

In yet another soap opera-esque twist to the story, Richard Aicardi has a twin brother, James, who was at the same party and whose presence may have confused investigators and witnesses. He has not been charged.

Mr. Aziz’s family has filed a civil suit against the defendants and others allegedly involved in the assault.