Photo courtesy of the Boston Fire Department

Boston firefighters responded Saturday to the second fire in five weeks at the One Congress office tower in downtown Boston.

In the latest incident, fire broke out in a utility shaft on the sixth floor of the 600-foot tall skyscraper, Fire Commissioner Paul Burke said at a media briefing. Construction workers evacuated the structure at the corner of Congress and Sudbury streets shortly before noon.

Boston firefighters spent over seven hours at the scene as part of a four-alarm response. Firefighters cut through ductwork to check for extension, according to the department’s posts on Twitter. Three firefighters were hospitalized with minor injuries.

The future State Street Corp. and Intersystems Corp. headquarters broke ground in mid-2019 and has been scheduled for completion in December. Construction resumed this morning, developer HYM Investment Group said, and is scheduled for completion in 2023.

The cause is under investigation, Burke said.

On June 24, construction was halted after a fire broke out on the 20th floor of the 1 million-square-foot tower.

Construction at the mixed-use Bulfinch Crossing redevelopment of the Government Center garage property also has been plagued with structural problems that caused the shutdown of the MBTA’s subway lines beneath the site, and a partial collapse that killed a construction worker in March during demolition work at the garage structure.

State transportation officials shut down the MBTA Green and Orange lines for emergency repairs to the water-damaged column, which supports the garage structure, before reopening them on June 26.