SAN FRANCISCO -- A 22-year-old man is in custody Sunday for allegedly fatally stabbing his father and seriously injuring his stepmother at their San Francisco home late this morning, a police sergeant said.

The stepmother called police at 10:23 a.m. to report a family disturbance at her home at 49 Ceres St., according to San Francisco police Sgt. Wilfred Williams.

Responding officers found the father dead at the home and the stepmother suffering from life-threatening stab wounds, Williams said.

The son, who allegedly left the home after the stabbing, was found at a Walgreens store several blocks from his father's house when a person at the store called police to report the man was acting suspicious, Williams said.

When officers linked the man to the stabbing victims, they realized he was allegedly responsible for the father's death and stepmother's injuries, according to Williams.

Williams said a motive for the stabbings remains under investigation, but they appear to have stemmed from "some type of argument."