MBTA OFFICIALS said they have narrowed their investigation into the latest safety problem with new Orange Line vehicles and plan to brief the transit authority’s board starting next week on strategies for dealing with the manufacturer.

The manufacturer, the Chinese company CRRC MA, is facing pressure from the T to produce new Red and Orange Line vehicles faster and with fewer bugs, but frustration is building.

As first reported by the Boston Globe, a December 22 letter from the MBTA to CRRC roundly criticized the Springfield-based assembly facility.

“Given the breadth, number, and age of chronic quality issues that have remained unresolved, it becomes abundantly clear that CRRC MA’s management has completely abandoned its core responsibilities and commitment to lead, monitor, and support quality management,” the T letter said. “This situation has already caused major disruptions, rework, and delays in production and delivery of Orange and Red Line cars.”

Jeffrey Gonneville, who is serving as the interim general manager of the MBTA until Gov. Maura Healey names her own appointee, said the T needs “good quality vehicles” and hinted he would be more forthcoming next week about steps needed to get this “incredibly complex project” back on track.

“One of the things we’re going to be focusing in on is really continuing to work on building internal and external trust and credibility in our organization,” he said. “Part of that is really talking and thinking about transparency on a number of issues.”

At Thursday’s meeting of an MBTA safety subcommittee, T officials provided an in-depth update on a safety issue that first surfaced during a routine inspection of an Orange Line vehicle on December 26.

The inspector discovered a grounding strap had disconnected and was hanging below the vehicle. All of the new Orange Line vehicles were subsequently checked and similar problems on nine vehicles prompted their removal from service. Eleven axles were also replaced because of electrical arcing problems related to the grounding straps rubbing up against the axles.

The issue was first disclosed to the public on December 30 and scant details were provided.

At Thursday’s safety subcommittee meeting, T officials said all of the Orange Line vehicles have been returned to service during morning and evening rush hour but individual train sets are removed from service on weekdays at midday and at night and on weekends so each vehicle can be inspected once a week.

Eric Stuthoff, the T’s chief engineer and acting chief operating officer, said the precise cause of the problem hasn’t been identified, but he said the focus is on the connectors where the grounding straps attach to the vehicle. He said the connectors appear to be undersized and prone to cracking.

Stuthoff said engineers from the MBTA and CRRC are working on correcting the problem.

He said the weekly vehicle inspections are sufficient to prevent safety problems, but he indicated regular inspections are necessary until the problem can be definitively identified and addressed. He said that could take until early February.

“If allowed to go unmitigated for too long, you could create metallurgical changes to the metal of the axle that could result in cracking, creating a long-term safety risk,” he said.