Firefighters found a cap and a cellphone next to the body but nothing to indicate why he climbed the tower early Friday, said department spokesman Kyle Moore.
There was no one at the scene who said they knew the man, Moore said. The King County medical examiner's office took the body to determine his identification and cause of death. Seattle police also are investigating.
The fire department initially received a report about 12:30 a.m. Friday of what appeared to be a transformer fire on the tower that carries 120,000-volt lines across the Lake Washington Ship Canal in the Fremont neighborhood.
It was followed by a report of witnesses seeing a flash of crackling sparks and a man falling.
Seattle City Light said it was too dangerous for firefighters to rush up the tower, Moore said. They were able to call in a Snohomish County helicopter for assistance. It spotted the body on the platform with no signs of life.
City Light turned off the power through the lines so a fire department technical rescue team could climb up and recover the body from the platform about 150 feet off the ground. The body was lowered to the ground before 7 a.m. in a basket.
The utility was able to switch electricity so only one customer was out of service during the recovery, said spokesman Peter Clarke.
It was the first such mishap Clarke could recall in his 13 years with the city utility. It would be possible for a "very determined" person to climb the lattice of metal in one of the tower legs, he said.
"We're going to be reviewing that to see if we can make it less easy," Clarke said.