SAN FRANCISCO -- Homicide investigators are trying to determine today what led a 45-year-old San Francisco man to stab another man to death inside a Richmond District parking garage Wednesday afternoon, a police spokeswoman said.

Ryosuke Yoshioka, 59, of San Francisco, was attacked in the parking garage located above the OfficeMax at 3700 Geary Boulevard at about 2:45 p.m., according to authorities.

He was pronounced dead at the scene, the medical examiner's office said.

According to police spokeswoman Sgt. Lyn Tomioka, investigators still are not sure what precipitated the attack, which was witnessed by at least three people.

The bystanders were able to detain the suspect in the stabbing at the scene until officers arrived and arrested him, Tomioka said.

The suspect, Peter Fong, remained in custody today after being booked for murder, according to Tomioka.

Fong bears "no relation" to current Police Chief Heather Fong, Tomioka emphasized.

A manager at the Office Max said today he had been informed of the stabbing by police, but had no knowledge of the incident at the time.

Any additional witnesses to the attack are asked to contact the Police Department's homicide detail at (415) 553-1145 or the confidential tip line at (415) 575-4444.

The stabbing is the first homicide reported by San Francisco police this year.