A missing person case from 2009 led to a backyard in La Puente, where investigators were seen Thursday digging into a small patch of earth in search of human remains.
Authorities told reporters at the scene little about the dig and refused to say who the missing person was because it might compromise a potential homicide investigation.
Video from the scene showed investigators drilling a hole deep into the ground and meticulously removing dirt that was put into buckets.
The person investigators from the L.A. County sheriff's and coroner's departments are looking for is not believed to have been a resident at the home in the 700 block of Elsberry Avenue, officials said.
The family who lives there is cooperating with the investigation.
Authorities would not say who tipped them off to the possibility that human remains could be in the backyard.
A coroner's official said the dig had been planned for two days. It began about 7 a.m. Thursday.