BOSTON — A 27-year-old axle on a Red Line car fractured and caused a June 11 derailment, MBTA officials announced Monday, offering a clearer picture of the problem that has led to months of delays and headaches on a train line that transports hundreds of thousands of commuters

The train reached that condition as a result of gaps in inspection schedules and missed warning signs, investigators found. While axle damage sometimes occurs on heavily used vehicles, the train in question was a matter of weeks away from a semiannual inspection that would have detected it. Crews performing preventative inspections roughly every three months also had not been checking the ground ring, a related part that helps keep the train electrically grounded, that was damaged and caused the axle to become fragile.

MBTA Deputy General Manager Jeff Gonnville said the transit authority had no records of any prior incident with the ground ring. Moving forward, the MBTA will work to prevent those problems from arising again. Ultrasonic inspections will now be conducted annually instead of every two years, and regular inspections will include close examination of the ground ring assembly for any damage that could emanate out to the axle or other parts.

"We have impacted a lot of people on the Red Line over the last several months," Gonneville told reporters at a Monday briefing. "It is our obligation to make sure this doesn't happen again." Gonneville told reporters Monday morning that the MBTA knew early on that a fractured front axle, in service since 1992, on the third car of the Red Line train caused it to careen off the rails just outside of the JFK/UMass station in June.

The News Service inquired in early July about a broken axle on the derailed train set; at the time T officials said only that the agency was still working to determine the root cause of the derailment.

As investigators probed how exactly the axle was broken — and how it sustained damage after its mid-2017 ultrasonic inspection showed no problems — they discovered another problem with the ground brush assembly.

The brush receives electricity from the motor and transfers it onto a ring wrapped around the axle, which then directs it through the wheels and into the rails. A functioning ring is made of slick, clean steel, sealed off from the environment to prevent being worn down.

But the ring next to the broken axle was covered in pits and scuffs, disrupting the clean electrical connection and sending what Gonneville likened to lightning on a smaller scale arcing across the axle. How exactly the ring came to be so corroded remains unclear, Gonneville said. A December 2018 preventative maintenance inspection, one conducted roughly every three months, noted that the assembly itself was missing the cover that keeps it sealed.