REDWOOD CITY (CBS SF) — A plumber pleaded no contest to second-degree murder on Tuesday for beating another man to death with a metal bar in a South San Francisco gym in 2014, prosecutors said Wednesday.

Kenneth Osako, 47, entered the plea in the courtroom of Judge John Grandsaert and faces a minimum of 15 years to life in prison when he is sentenced on April 25, according to the San Mateo County District Attorney’s Office.

The victim, 44-year-old Diego Galindo, was at Bally Total Fitness gym at 180 El Camino Real at about 9:30 p.m. on Sept. 17, 2014, when Osako approached him from behind, struck him three or four times with an 18-inch metal bar used to hold weights, and fled the area, prosecutors said.

Galindo was taken to San Francisco General Hospital but died from his injuries the next day. Witnesses identified Osako as the assailant and he was arrested the day after the attack.

Witnesses heard no exchange between the two men before the beating, according to prosecutors. The motive for the attack has not been discerned, but prosecutors said Galindo had made a joking request to Osako’s girlfriend to take a ride on his motorcycle a day prior.

Osako’s attorney, Steven Chase, had previously argued that Osako never intended to kill Galindo and said Osako was “horrified” that he died.