The medical examiner has identified a leg bone found inside a boot that washed up onshore near a San Diego-area beach Wednesday as a human tibia.

Josh Mercado was riding his bike along the beach at 3:30 a.m. when he found a black rubber boot with a bone inside.

"It was a big bone," he said of the 11 and a half inch tibia.

"I'd say the bone was completely intact," Mercado told NBC 7. "If it wasn't the femur, it was definitely the tibia. There was a sock in there as well."

The boot was approximately 70 yards south of the beach access stairs off Carlsbad Village Drive and Ocean Street, police said.

Mercado said he saw the boot as he rode on the beach and when he picked it up, he noticed some resistance.

As he tipped the boot upside down by the toe, the bone came out, he said. He said he took a few steps back and called 911.

Oceanside Harbor Patrol sent a boat to search the beach after daylight. No other bones were found, police said.

Officers took the boot to the San Diego County medical examiner's office for further investigation where they confirmed the bone was a partial left tibia belonging to man about 5'8" to 5'10" with a muscular build. The bones had probably been there about a month officials said.

The medical examiner believes the cause of death is likely drowning but the investigation is ongoing.

The victim's family has not yet been notified.