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Oakland police officers investigate a crime scene at an apartment complex on the 1800 block of 34th Avenue in Oakland, Calif., on Friday, Feb. 7, 2014. An elderly woman was killed apparently during a home invasion robbery, police said. (Ray Chavez/Bay Area New Group)

OAKLAND -- A 77-year-old woman was found dead Friday afternoon in what police say was a home-invasion robbery.

Officers responded to an apartment on the 1800 block of 34th Avenue around 2:15 p.m. after a caller asked for a welfare check on the residence, police said. When they arrived, they found the woman, later identified by police as Susan Fong, and a 56-year-old man inside.

Fong, who was unresponsive when officers arrived, was declared dead by paramedics a short time later. The man was not injured.

Police said Fong's cause of death could not be determined as it did not appear she suffered either a gunshot wound or blunt force trauma.

There were signs of forced entry at the apartment, police said, but investigators were still trying to determine Friday evening if anything was taken from the home.

Police said that Fong owned the four-unit apartment building where the robbery took place, and had lived there for at least 30 years.

Fong and the male victim are related, police added, but they would not elaborate. Investigators are questioning the man, and he is being cooperative, police said¿.

The death comes just over a week after an 81-year-old woman was shot and wounded by an intruder during a home-invasion robbery Jan. 28 in her Dimond District home.

Anyone with information on the incident is encouraged to call the Oakland Police Department's homicide unit at 510-238-3621.

Police and Crime Stoppers of Oakland are offering up to $20,000 in reward money for information leading to the arrest of the killer. Anyone with information may call investigators at 510-238-3821, a tip line at 510-773-2805 or Crime Stoppers at 510-777-8572.

Staff writers Katie Nelson and Harry Harris contributed to this report. Follow Kristin J. Bender at Twitter.com/kjbender.