Metro will extend lower levels of service for at least another week, saying the transit agency is sketching out a path to allow its 7000-series rail cars — stricken by problems with wheel assemblies that led to a derailment this month — to return to use.

Metro General Manager Paul J. Wiedefeld said the basic service Metro has offered since pulling 60 percent of its rail cars from service will extend through Oct. 31. In his first public comments in nearly a week amid a National Transportation Safety Board investigation, he said the transit agency hasn’t set a timetable on boosting rail service back to normal levels.

“It’s been a difficult week for people who depend on Metro,” Wiedefeld said Friday, his comments coming hours before Metro was forced to evacuate a train that afternoon for the second time in 11 days.

It capped a tumultuous week for a transit agency that had been on the rebound in recent weeks as more riders were turning to transit amid a gradual return to office work. The Blue Line train derailment, NTSB probe and basic levels of service frustrated riders weeks after the transit agency enacted lower fares and more service in hopes of luring them back.

Federal investigators indicated during a Monday briefing the agency knew of wheel assembly problems with Metro’s 7000-series cars since 2017 but did not remove them from service or report the issues to the independent Washington Metrorail Safety Commission. Wiedefeld said Friday the wheels were under warranty and being replaced over a span of years. But until this year, only isolated cases were discovered among the thousands of inspections Metro carries out annually.

He acknowledged that when problems ticked up dramatically this year, more should have been done. NTSB Chair Jennifer Homendy said Monday that Metro identified two failures each in 2017 and 2018, four in 2019 and five in 2020. Before the derailment, the agency had found 18 failures this year, and inspections last week turned up 21 more. “That is definitely where that should have been raised much sooner,” he said.

Metro officials said they are looking at the process for how to return the 7000-series cars to service. The question of how to get the cars safely back in service was being tackled separately from the issue of determining the root cause of the defects, Wiedefeld said, holding out the promise of a shorter disruption to Metro service. He said components of the plan include determining how often rail cars will need to be inspected, then conducting tests to ensure the new inspection regimen is effective.

David Mayer, chief executive officer of the Washington Metrorail Safety Commission, an independent government agency that oversees Metrorail safety, endorsed that approach Friday. He said determining the root cause might take time, describing the issue with the cars as a “progressive defect” that gets worse over time.

“The main thing I think needs to be focused on in the short run when we’re talking about returning cars to service is making sure we all have a very good scientific technical grasp on the rate of progression of the defect so that we can set a safe inspection interval and prevent future derailments,” Mayer said in an interview Friday.

He said Metro could use data from inspections it has carried out in recent days and measurements taken during tests of trains to determine a safe interval. The commission this week ordered Metro to create a plan to detect and assess the wheel assembly problem, as well as come up with a process that can safely return the cars to service.

“Metro is working, we know, to create a plan as we required in our order, and we would expect to find elements like that in a plan,” he said.

The 7000 series is Metro’s newest, largest and most advanced train set, with greater ability to operate trains with eight cars instead of six. Transit officials said an agreement with the NTSB restricts them from speaking about the investigation. Manufacturer Kawasaki Rail Car, which has joined the NTSB probe, has not responded to requests for comment this week.

NTSB investigators are trying to determine why wheels on nearly two dozen cars shifted, putting trains at greater risk for derailments and other incidents. An investigation team that includes Metro officials and the safety commission has disassembled and inspected wheel sets at a Metro maintenance facility this week.

No one was injured in the Oct. 12 derailment, which had prompted the evacuation of 187 passengers outside the Arlington Cemetery station, but it led the safety commission to pull all 748 of Metro’s 7000-series cars out of commission Sunday night.

The suspension has forced the transit agency to run the rail system using 40 train sets, operating trains every 15 to 20 minutes on the Red Line and every 30 to 40 minutes on other lines. Silver Line trains are operating between the Wiehle-Reston East and Federal Center SW stations. The service cuts have created some of the most crowded Metro conditions since the start of the pandemic.

Wiedefeld said he recognized problems in recent days could hurt the public’s confidence in the system, but said he believed trust could be recovered. “We’re not hiding anything,” he said. “We want to be as transparent as possible.”

Friday’s announcement of an extension in reduced service came two days after Metro’s board announced it will hire outside experts to scrutinize procedures, an effort partly aimed at restoring public confidence.

Paul Smedberg, the board chairman, who appeared alongside Wiedefeld in a virtual briefing Friday, said the board continued to have confidence in Metro’s leadership. “We’re behind the entire team,” he said.

Smedberg acknowledged the board had concerns about not being informed of wheel assembly issues sooner, but said he was reserving judgment on whether the board should have been briefed.

D.C. Mayor Muriel E. Bowser (D) said in a statement that Metro this week “failed to execute upon safety in a way that meets our expectations and that has impacted reliability and capacity.” She said the city needs a fully functioning transit system for workers, children and visitors.

Democratic Sens. Ben Cardin and Chris Van Hollen of Maryland and Mark R. Warner and Tim Kaine of Virginia wrote Wiedefeld on Thursday night calling on him to share more information about the transit agency’s recovery plans and safety efforts.

“We will be eager to hear from you in a timely manner on your plans to address the specific safety concerns associated with last week’s derailment, to restore public confidence in your organization, and to embed safety more effectively into your organizational culture — a repeated focus of our discussions with you since you took the helm of WMATA six years ago,” they wrote.

Hours after Wiedefeld’s comments on the transit agency’s recovery, Metro found itself evacuating passengers from a disabled train in another headache for passengers. About 100 riders were removed after the 3000-series train broke down about 100 feet from the Gallery Place station’s platform, said Metro spokeswoman Sherri Ly.

Rail service on the Green and Yellow lines between Mount Vernon Square and L’Enfant Plaza was briefly suspended, but no one was injured.

Metro is operating with 268 rail cars, or about 22 percent of its roughly 1,200-car fleet. Nearly 250 are from its 2000 and 3000 series lines that entered service as far back as 1982. Other cars are in storage or undergoing repairs, and Metro mechanics and technicians are working to get them back into service.

The transit agency’s stock includes 22 rail cars from Metro’s 6000 series, up from 16 earlier this week. The cars had been pulled from service in November after two train separations on the Red Line last year. Metro said rail cars are being phased in after an inspection process. Between 10 and 15 percent of available rail cars are typically out of service on any given day for maintenance or repairs.

Business leaders have urged a quick resolution to Metro’s service woes, saying lengthy service problems are more likely to push riders to find alternatives. “How long will this last? That’s a big concern,” said J.B. Holston, chief executive of the Greater Washington Partnership, which consists of chief executives of many of the region’s largest companies.

In an interview Thursday, he pointed to the disruption Metro’s service levels have caused even as much of the region is working from home and ridership is about one-third of normal levels.

He said it is important for Metro to be as clear as possible with riders about what to expect. “Suddenly we’re back where we don’t want to be,” Holston said. “Anything and everything that everyone can be doing to get this resolved is in the direct interest of our economy.”