San Francisco’s new Transbay Transit Center will remain closed at least through the end of next week, officials said Wednesday, after yet another cracked beam was discovered during an overnight safety inspection.

The $2.2 billion hub for buses and eventually trains, which opened just last month as the flashy centerpiece of city infrastructure, was closed abruptly Tuesday afternoon after a fissure was spotted in a beam that helps hold up the sprawling complex.

The initial tear runs about 2½ feet long and 4 inches deep through the bottom of a 60-foot-long beam that supports both the center’s celebrated rooftop park above and a bus deck below, officials said. The beam is located over Fremont Street, between Mission and Howard streets. The second crack is in a parallel steel beam that also crosses Fremont Street. It was described as slightly smaller.

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Representatives of the Transbay Joint Powers Authority, which built and operates the transit center, said Wednesday they didn’t know the causes of the cracks, but they remained concerned about the potential for the beams to fail. Fremont Street, which passes under the center, also is scheduled to stay closed through Oct. 5.

“We will not open the transit center or Fremont Street until we are certain the issue is 100 percent rectified,” said Mark Zabaneh, executive director of the TJPA.

The transit center is next door to Millennium Tower, a 58-story residential complex that has sunk 18 inches since opening in 2009. Officials said issues in the neighboring buildings were not linked.

Inspectors have been coursing through the three-block-long depot since workers installing ceiling panels noticed the first crack Tuesday morning. They said they’ve found no problems elsewhere in the transit center, including a section over First Street that has a design similar to the Fremont Street span, but will continue to evaluate the entire building.

TJPA officials said they’re coordinating with the transit center’s primary contractor, a joint venture of Webcor and Obayashi, and structural engineering firm Thornton Tomasetti, which also worked on the building, to perform the inspections. They also plan to bring in independent engineers to reassess the facility’s design.

“I want to be very clear that this is a localized area and that we are looking at other locations just to make sure there are no other issues at the transit center,” Zabaneh said.

The American-made steel in the cracked beams was produced as part of a $189 million contract that Skanska USA Civil West of New York had with the Transbay Joint Powers Authority, records show. The beams were fabricated by Herrick Corp. in Stockton, according to the TJPA.

The beams are among more than 22,000 tons of steel that make up the skeleton of the building, according to construction documents.

TJPA officials said there could be any number of reasons for the cracks in the steel: a fabrication problem, an installation error, too much weight on the beam or a hitch in the initial design of the building.

The cracks were not present when the beams were inspected after their installation or when the beams were sprayed with a fireproofing coating in summer 2016, officials said. But there’s no telling when they might have developed after that.

According to Dennis Turchon, the project’s senior construction manager, the workers who noticed the first fissure were a few feet from the crack. From a distance, it would have been easy to miss.

TJPA officials said they’re planning to shore up the beams by bringing in additional support props, which they hope to have in place by Oct. 5 so they can reopen Fremont Street. The ultimate goal is to mend the steel beams so they’re fit to function on their own.

San Francisco Mayor London Breed pledged Wednesday to move the process along as swiftly as possible.

“The Transbay Transit Center is too important for our city and our regional transportation system not to act quickly to have definitive answers for the public, and someone needs to be held accountable once the cause is determined,” she said in a statement.

City officials, however, don’t have direct authority over the transit complex as the TJPA is a joint group created under state law.

The cracked beams were not the first problem for the transportation center and its modern design, with a lacy facade and roomy, well-lit foyers inside. The walkway that loops around the rooftop park — known as Salesforce Park — has recently begun to crumble. The center and park have been open since Aug. 12.

The publicly funded building has been in the works for nearly two decades, with groundbreaking in 2010. Envisioned as the “Grand Central station of the West,” its full potential won’t be realized until planned connections with Caltrain and high-speed rail service are made. It could be a decade or more before those ties are completed.

City officials on Wednesday advised motorists to avoid driving downtown as traffic backed up with the closures of the transit center and Fremont Street. It didn’t help that one of the city’s largest annual conferences, Dreamforce, is taking place this week, drawing 170,000 people.

AC Transit, SamTrans and Muni buses that serve the transit center have resumed service using the Temporary Transbay Terminal at Howard and Main streets, where they had operated during construction of the new building.

Chronicle staff writer John King contributed to this report.

Kurtis Alexander and Evan Sernoffsky are San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com esernoffsky@sfchronicle.com Twitter: @kurtisalexander @EvanSernoffsky