"They were facing the ocean," Encinitas Fire and Marine Safety Capt. Larry Giles told The Washington Post. "So, it came from behind them and basically overtook them and crushed them."
First responders and rescue dogs searched the rubble for more possible victims Friday night.
Grandview Beach is close to San Diego and is known as a popular spot for surfers, according to KSND-TV.
The cliff collapse left nearly 20 yards of debris material on the beach, Encinitas Fire Chief Mike Stein told CNN. Beachgoers were "recreating right underneath the bluffs," Stein said.
Natural erosion can make it difficult to predict when parts of a cliff will break off, Giles said.
"Encinitas lifeguards here along with the state park lifeguards, we warn people excessively throughout the year ... we tell people over and over and over to stay back from these naturally eroding cliffs," he said.