Columbus fire officials said they think workers in an elevator shaft of the historic Hartman Building at E. Main and S. 4th streets sparked the blaze with a welding torch and left without realizing what had happened.
A passer-by noticed the flames about 7 p.m. and called firefighters, Battalion Chief Robert Schmidt said.
Four men preparing drywall on the first floor continued to work, not realizing a fire was upstairs.
"A gentleman off the street ran in yelling, 'The building's on fire!' '' said Ralph Webb, one of the workers.
"I remember saying: 'No way. You're kidding.' You couldn't smell anything.''
Moments later, they saw fire engines.
"We grabbed our tools and ran,'' Webb said.
The 72,000-square-foot Hartman Building, a former hotel built in 1898, has been undergoing a $3 million renovation to house several technology companies.