Four levels of inspections and plans failed to prevent or uncover a construction flaw that resulted in the cracked girders that forced the closure of the Transbay Transit Center 5½ months ago, the agency’s executive director said Thursday.

Mark Zabaneh, head of the Transbay Joint Powers Authority, said three teams of quality-control inspectors working for contractors didn’t discover that a necessary grinding process, used to eliminate small cracks associated with holes that were cut into the girders, was not performed and that the authority’s own spot inspections also missed the oversight.

Those micro-cracks developed into larger cracks and, finally, a large fissure that ripped through two large steel girders that support the building. Similar girders over First Street have not cracked.

“The execution was not done properly, and that is something we are looking into,” Zabaneh said Thursday after an authority meeting. “It’s an area of great concern for us.”

The failure of the quality-control and quality-assurance processes to detect the fracture-inducing error prompted the authority to order engineers and architects to pore over tens of thousands of drawings and inspection documents to make sure no other major potential flaws were overlooked.

A pair of outside engineers told the Transbay authority’s board Thursday that the two girders that support the three-block-long transit hub where it crosses Fremont Street fractured because of an unusual confluence of factors, including the relative strength of the steel, the design of the structure and the fabrication process.

Robert Vecchio, of LPI, a New York laboratory that analyzed metal samples from the failed girders, pointed toward the cutting of holes, known as welding access holes, in the beams as the source of the problem. The holes over Fremont Street were cut in the beams before they were welded. The welding caused tiny cracks to grow and then fracture. Holes in the girders over First Street were welded before cutting and did not crack.

Grinding the edges of the holes smooth before welding can eliminate the micro-cracking, Vechhio said, and would have prevented the Fremont Street girders from fracturing, he said.

Zabaneh said after the meeting that quality-control teams for Herrick, the steel fabricator; Skanska, the steel installer; and Webcor-Obayashi, the general contractor, failed to notice the oversight.

So did Turner Construction, a quality-assurance contractor that conducted occasional “spot” inspections for the authority to make sure the contractors were following plans and meeting standards.

Representatives of the companies were not immediately available for comment Thursday.

The $2.2 billion transit hub, rooftop park and retail center was open just six weeks when workers discovered cracks in the two critical girders on Sept. 25. Since then, engineers have struggled to discover the cause of the fractures and devise and complete repairs.

The repairs involve sandwiching each of the girders with thick steel plates joined with 224 steel bolts. The steel was being fabricated Thursday, said Transbay construction manager Dennis Turchon, and is expected to arrive shortly.

Transbay officials expect the repairs to be completed by June but say they can’t set a reopening date for the transit center until they complete their inspection of construction records and determine whether further physical inspections — or repairs — are required.

Engineers and designers have reviewed about 85 percent of the records, Transbay officials said, and have found no reason for concern.

“We want to make absolutely sure the building is safe and the structure is sound before we let the public back in,” Zabaneh said.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan