After a six-hour manhunt, authorities late Wednesday captured a man sought as a "person of interest" in a triple stabbing that left a 1-year-old boy and a 48-year-old woman dead on Wednesday at an Alum Rock neighborhood home, where relatives of the victims sobbed openly in the street.

U.S. marshals and San Francisco police detained Ruben Ramirez, 27, about 9:15 p.m. in San Francisco, even as authorities refrained from calling him a suspect in the killings and the stabbing of a 3-year-old girl, who was in stable condition at a hospital.

"We have no other persons of interest at this time," said Undersheriff John Hirokawa of the Santa Clara County Sheriff's Office. The slain woman, whose name was not released, was the children's grandmother and was baby-sitting them, he said.

He did not disclose Ramirez's relationship to the children, but relatives said he was their uncle and the dead woman was his mother.

The stabbings capped an unusual day of violence for the South Bay, which saw three homicides, including a woman who was found dead inside a room at the Holiday Inn Express Motel on Monterey Road in San Jose about 1:20 p.m.

Wednesday's deadly scene on Viewmont Avenue near Alum Rock in unincorporated Santa Clara County unfolded about 3:30 p.m., when a family member arrived at the "unusually quiet" house and found the three stabbing victims.

Ramirez apparently lived at the home and has a history of mental illness. According to Santa Clara County Sheriff's Office spokesman Deputy Kurtis Stenderup, authorities had previous contact with him for reasons they did not immediately disclose.

According to televised media reports, Ramirez also frequented homeless shelters.

Arturo Maries, pastor of the Las Palmas Jehovah's Witness Congregation on South White Road, said the woman killed comes from a large family who attended his church regularly.

"They are a normal family; they go to church all the time except for the one who has a sickness," said Maries, referring to Ramirez.

Maries also said that family members had voiced concerns that Ramirez had not been taking his medication for his mental condition. Sheriff's officials did not comment further on Ramirez's mental illness.

Even as the sun fell, a line of spectators stretched down Alum Rock while sheriff's helicopters circled over the scene, which remained cordoned off by police tape.

Sunnyvale police assisted in the manhunt, joining sheriff's deputies in the search for Ramirez.

Contact Eric Kurhi at 408-920-5852. Follow him at Twitter.com/erickurhi.