The bodies of three men and two women were found Sunday in the space between commercial buildings and Interstate 405 in Long Beach, police said.
An anonymous call from a pay phone at about 8:30 a.m., had directed police to the scene.
"We're still trying to find out who that person was, we would love for them to come forward," Long Beach Deputy Police Chief Robert Luna said. "We believe they have more information than what they gave us."
Every day thousands of people drive by the scene, near where Interstate 405 meets Interstate 710, but the encampment is hidden by heavy brush and the crime had no apparent eyewitnesses.
"The way they were situated, it was perfect in that no one can find the crime scene or see it," Luna said.
Some neighbors reported hearing gunfire and yelling at about 12:15 a.m. Sunday.
"I heard the gunshots first, and then I heard the screaming," neighbor Tippi Briggs told KNBC-TV. "I heard a guy yell 'come on, let's go, get in the car, let's go' and then I heard the car, it just sped down the street."
Detectives have not determined a motive or identified a suspect. The bodies on Monday morning were at the county coroner's office, where investigators were trying identify the victims and determine how long they had been dead.
Some of the victims had been shot more than once, said Ed Winter, assistant chief in the Los Angeles County coroner's office.
Dozens of detectives combed the area for evidence all day and into the night on Sunday.
"Because of the rains last night and the heavy foliage in the area where the crime occurred, the investigative process will be slow and methodical," Luna said.