For the first time in years, commuters in the Washington region who ride Metro four days a week outnumber those who ride every weekday — a subtle but significant ridership shift that transit officials fear may have long-term implications for the transit agency.

According to data released Tuesday by Metro’s Office of Planning, there has been a steady decline since 2013 in the average number of monthly trips taken by commuters carrying SmarTrip cards — down from 20 trips per month to 18. And data analysts contend that only about 30 percent of Metro’s recent ridership losses are due to people abandoning the system because of reliability and problems with on-time performance.

Instead, the rise of teleworking may have a more significant impact on ridership, confirming theories raised by transit officials in recent months: Rail ridership during morning peak-period hours on Fridays — a typical teleworking day for the federal government — is usually 15 to 20 percent lower than on days in the middle of the week.

That may play an outsize role in the steady ridership decline that Metro has seen in recent years. And it’s a trend that weighs heavily on Metro General Manager Paul J. Wiedefeld. Metro must improve service and reliability, he said, but teleworking is a threat to the agency’s finances — and something over which it has no control.

“It’s not even four days a week now,” Wiedefeld said at a recent meeting, citing new numbers from the federal government on rates of teleworking. “People are coming in twice and three times a month. I can’t change that. That’s going to be what it is.”

Monitoring how riders use the system — and on what days of the week frequent commuters travel — is a relatively new ability for Metro, made possible by the introduction of SmarTrip cards that allow Metro to monitor individual cards to learn about the frequency and timing of use. Metro officials say they intend to use that data to drill down into details that could explain exactly why ridership is ebbing — and how to stem the decline.

The data also paints a sobering picture of the future of Metro: Though officials continue to hope that improving reliability and rehabbing the system’s infrastructure may help win back some riders, there is a growing acknowledgment that the numbers may never return to the boom-time ridership of the early 2000s, when it increased dramatically year after year.

Ridership growth reached its apex in 2008, when it peaked at 750,000 average weekday boardings. Now it hovers just over 600,000 average weekday rides.

Metro’s continued lackluster ridership numbers signal a fundamental shift for the agency’s data analysts and planners, who for years have relied on a simple theory: If the local economy is improving and the Washington region is growing in population, then Metro ridership will also grow.

But now, analysts indicated in the report, it’s clear that the old formula doesn’t work anymore. Anticipating growth on Metro has become a lot more complicated.

“[Metro] needed to rebuild its capacity to monitor and forecast ridership using new data and methods,” analysts say in the report.

They also suggest that ride-share apps such as Uber and Lyft play a major role in Metro’s ridership woes, particularly when it comes to evening rides after rush hour — when longer average waits between trains make door-to-door car service even more attractive.

And a decrease in federal transit benefits several years ago may also have resulted in some portion of the federal workforce opting to use modes of commuting other than public transit.

However, staffers were blunt in noting that unexpected delays, service interruptions, peak-period meltdowns, ill-timed train offloadings and SafeTrack maintenance projects are a significant part of the ridership problem.

“We realize that it will take some time to regain the trust and confidence of customers needed to return to the system,” the planning staff wrote in the report.

The report, which will be presented at Thursday’s board meeting, said large-scale efforts might be the best solution for the ridership decline long-term. It recommends, for example, that regional leaders take more proactive steps to encourage ridership, such as investing in the construction of housing developments near Metro stations.

They also push for infrastructure improvements that can help public transit operate more efficiently and attract riders with shorter, faster rides — such as installing bus-only lanes and instituting traffic-signal prioritization for buses, as well as fostering pedestrian-friendly neighborhoods that encourage people to walk to their nearby Metro station rather than drive.

Aimee Custis, deputy director of the Coalition for Smarter Growth, said Metro’s new data suggests that regional leaders are responsible for doing more to help the transit agency regain popularity.

“There are things that must absolutely be fixed in-house at Metro,” Custis said. “But we also must deal with things that are outside of Metro’s control, or we won’t solve this problem.”

Wiedefeld said recently that he remains hopeful that the long-term prospect of development and construction in the region means that boom times may still be to come for Metro. In particular, he said, the completion of the second stage of the Silver Line could bring thousands of new daily riders to the system.

“It will still be a few years before some of that occurs,” Wiedefeld said. “But as they get rid of those car lots and stuff and build 20-to-30-story buildings . . . that will start to drive the numbers up.”