SAN DIEGO (CNS) - A 22-year-old accused of beating a 72-year-old man on a city bus and allegedly stabbing his father to death a day later in a City Heights apartment pleaded not guilty Thursday to murder and elder abuse charges.

Ismael Beltran faces 25 years to life in prison if convicted of killing 43-year-old Francisco Beltran.

Deputy District Attorney Meredith Claterbos said the defendant attacked 72-year-old George Myrick on a Metropolitan Transit System bus on Oct. 10, punching the sleeping victim about 15 times while holding a pen in his hand.

The next day, neighbors in the 3600 block of 42nd Street heard shouting and Francisco Beltran yelling "No son, no" in Spanish, and saw what appeared to be Ismael Beltran leaving the apartment. The victim was later found bleeding to death in the home.

Francisco Beltran, who was stabbed 20 times, died at a hospital, the prosecutor said.

Claterbos said investigators found a knife -- believed to be the murder weapon -- on top of a refrigerator in the apartment, and a bloody shoe print was located in the kitchen.

Ismael Beltran was taken by his mother to an Otay Mesa-area police station on Oct. 12, and he was arrested.

A print from the defendant's bloody shoe matched the shoe print found in the victim's apartment, Claterbos said.

A motive for the killing was not known.

Defense attorney Jane Kinsey told Judge Frederick Maguire that Ismael Beltran has been very ill recently and has been diagnosed with paranoid schizophrenia. Bail was set at $2 million.

A status conference was scheduled for Oct. 26 and a preliminary hearing for Nov. 2.