LOS ANGELES (CBS) — Police Wednesday looked to the public for help in identifying a man, who was involved in a bus fight that seriously injured another man.

Police say two passengers aboard a Santa Monica Big Blue Bus argued, then physically fought on the night of April 5. During the fight, Luis Contreras suffered a serious head injury and was fighting for his life, police said.

READ MORE: George W. Bush To Speak in Beverly Hills, Long Beach

“We believe that he [Contreras] may have accidentally stepped on the foot of another passenger. So an altercation occurred between the two of them,” said LAPD Officer Karen Rayner.

The bus driver pulled over at Westwood Boulevard and Rochester Avenue in Westwood and can be heard on a security video telling the men to get off the bus.

In the video it appears that punches were thrown, but it is not clear who started the fight.

“There is some pushing and shoving and the individual [Contreras] falls backwards out of the bus. And we believe that he may have hit his head on the curb,” Rayner said.

READ MORE: Biker With Several Felony Warrants Arrested In Westminster

They other man then walked over Contreras, circled back and then left, police said.

Contreras was rushed to the hospital in critical condition.

Several passengers on the bus witnessed the incident, but police say they have been unable to identify the second man involved in the fight.

“We’re asking for this individual who was involved to just come forward. We need to get his side of the story. We need to get more pieces to this puzzle,” Rayner said.

MORE NEWS: Tens Of Thousands Help In Massive Beach Cleanup Effort Saturday

Anyone with information about the fight or the identity of the man was asked to call LAPD West Bureau homicide detectives Armando Acero or Luis Carranza at (213) 382-9470.