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Authorities found the utility room in flames and called Southern California Edison to shut off the power as they proceeded to put the fire out.
They discovered the charred body of a man wedged behind an electrical panel, and tools used to steal copper wiring was found nearby.
"Bolt cutters, the lock was cut off the door, and a pair of wire snippers, and that's about all we found. Anytime you mess with electricity, it only takes a milliamp to kill you, and in this case there were hundreds of amps, and over 450 volts of electricity was instantaneous death," said Det. Rick George of the El Monte Police Department.
Authorities are investigating whether the victim was stealing copper wire at the time of his death. The utility room is the worst possible place to steal wire because power comes to the building through the utility room, and the active wires were still charged with electricity, police said.
A copper thief may have thought the building was a perfect target. It was vacant, dark, with no sign that power was flowing to a back electrical panel.
It's a risky attempt to cash in on copper's rising value, which is as much as $3.20 a pound in Los Angeles. At P&T Metal Recycling nearby, operator Kurt Rexius showed Eyewitness News the scraps that scavengers bring in.
Copper theft is rampant. Near the site of the latest electrocution, other businesses have chased off thieves.
"My supervisor surprised them and they jumped a fence. They stole all my rolls of wire out of my store rooms. They tried to get in our electrical panels here, but I got ahead of them and put a big lock on there," said Daniel Rupp, who works nearby.
Recyclers already require ID from collectors, but there's talk in some states of requiring fingerprinting as vandals destroy street lighting and public utilities.
What investigators say is most needed is more respect for electricity.
In Monday's deadly fire, the victim's body was burned beyond recognition. No fingerprints were available, and dental records may not even be available.
One of the few clues police have to identify the man is a white bicycle parked very close to the burned building, but police said they may have to use DNA analysis to identify the victim.