Police issued arrest warrants Monday for two men accused of a New Year's Eve attack on members of an all-male a capella group from Yale University.

Richard Aicardi and Brian Dwyer were charged with assaulting two members of the Baker's Dozen outside a party held in honor of the 16 student singers. Witnesses at the time said the trouble started after the vocalists, wearing sports jackets and ties, sang "The Star Spangled Banner."

Evan Gogel, one of the two most seriously injured chorus members, suffered a concussion when Aicardi, Dwyer and others repeatedly kicked him while he was on the ground, police said. Aicardi also was charged with punching Bakers Dozen member William Bailey as he tried to get away.

"This was a cowardly attack on defenseless victims," District Attorney Kamala Harris said in a statement. "We are going to hold accountable those who have been identified as responsible."

In the 10 weeks since the widely publicized attack, the San Francisco Police Department weathered criticism from families of the Yale students who were upset by what they considered a slow and inadequate response.

Police and Harris said the investigation was hampered because the young men could not identify their attackers. San Francisco investigators subsequently flew to Los Angeles and to New Haven, Conn. to gather more information.

Harris said Monday that she did not have enough evidence to charge anyone in the assault on a Baker's Dozen singer who suffered a broken jar during the melee, but that she had not ruled out the possibility that additional charges would be filed.

"If additional evidence emerges, we will move forward in that case as well," said DA Harris.