BOSTON (SHNS) – A top MBTA engineer said investigators have narrowed their probe to “material failure” as a possible explanation for power cable problems on the new Orange Line cars that prompted the agency to yank the vehicles from service last month.

MBTA Chief of Safety Engineering and Construction Steven Culp said Thursday that a Dec. 26 routine inspection of a CRRC-manufactured Orange Line vehicle discovered a “grounding strap that was disconnected and hanging underneath the train.”

The T performed similar inspections on the rest of the new Orange Line fleet and found multiple other instances of hanging grounding straps, some of which were in contact with vehicle axles. Culp said that as of Jan. 3, MBTA workers fixed the problem on all but four Orange Line cars and returned them to service. Each of the vehicles is now undergoing a weekly inspection. He added that an investigation remains ongoing and has not yet identified a probable cause for the problem, though Culp and Acting Chief Operating Officer Erik Stoothoff said an end connector holding down the strap may be too small for the cable being used.

“Presently, the strap connectors and cables are being assessed for a material failure as well as the routing of the cable. Those seem to be the primary engineering faults or causes for the issue that we have found,” Culp said at an MBTA board safety subcommittee meeting. “We are also looking at operational factors, but this is being somewhat discounted based on what we’re finding with the materials.”

The MBTA said in December when it identified the cable issue that it may have caused “some electrical arcing.” The T contracted with Chinese firm CRRC to build brand-new Orange and Red Line fleets.

In addition to multiple issues with the vehicles, the project has experienced substantial delays — partly fueled by the COVID-19 pandemic — and faced reports over poor work conditions in the Springfield factory.

MBTA Interim General Manager Jeff Gonneville said at Thursday’s subcommittee meeting he plans to address the full T board next week about “working with CRRC and talking through this project and some of the levels of complexity that we have with this project.”