A fire destroyed a machinery room Saturday in the 52-story Arco Towers, forcing dozens of weekend workers to be evacuated from the Downtown high-rise.
More than 100 firefighters put the late afternoon blaze out within 45 minutes. No injuries were reported.
"If it had gotten beyond the (machinery) room, it could have been very serious," said Alan Schroeder, the Fire Department's incident commander.
The room was on a mezzanine area between the 38th and 39th floors. "The room burned up," Schroeder said.
He said he did not know the nature of the machinery. No damage estimate was available.
Some firefighters speculated that the blaze may have been due to an electrical malfunction, but officially the cause remains under investigation.
Firefighters, responding to a smoke alarm at 4:15 p.m., discovered flames coming from the machinery room when they inspected the building, said Los Angeles Fire Department spokesman Brian Humphrey. No flames or smoke were visible from the outside, he said.
The 40 to 50 people working in the building were evacuated after security officers issued a warning.
Employees at the accounting firm of Ernst & Young on the 23rd floor said they smelled smoke as they made their way down a stairwell.
"My legs are wobbly," Judi Ward, a controller, said as she stood outside the building. "You really get the sense that you want to get out of there, hearing some of the tales of other buildings."
The fire was in the northernmost of Arco's two towers at 515 S. Flower St.
Times staff writers Ted Johnson and Ted Rohrlich contributed to this article.