The incident happened Tuesday afternoon at McKinley Elementary School on Paramount Boulevard and 68th Street.
Investigators say the man initially called police at 2:27 p.m. to report a home invasion, but when officers arrived on scene, they found him unconscious on school property.
Witnesses told officers that the man had climbed one fence to enter the school parking lot while carrying a sledgehammer.
He then attempted to climb a second fence to get onto the main campus when "he was pulled down and detained by a group of male adults," according to a Long Beach police statement.
"At some point, he became unresponsive," the statement added.
Officers say the man was unresponsive when they arrived. Paramedics soon arrived and confirmed he was dead.
It's not clear at this point how the man died or whether the encounter with the male adults involved violence. Police also didn't specify if the men involved in the confrontation were school staff members, security personnel or parents.
A large law enforcement presence was gathered at the school Tuesday evening, collecting evidence and speaking to potential witnesses. There were multiple evidence markers on the ground and the sledgehammer was still visible at the scene.
The school sent out a note to parents indicating the campus was locked down by law enforcement for what was described as a "medical emergency," and that the "campus is safe and secure." Classes will continue Wednesday as scheduled but there may be extra safety personnel on campus, the school said.
Another incident was reported at the same school last week, when a man was arrested after allegedly pointing a gun at a student. It's not clear if the two incidents are related.