An artist’s rendering of the interior of a planned Purple Line train in Maryland. (Purple Line Transit Partners)

A federal judge on Monday ordered Maryland officials to further scrutinize the potential impact of Metro’s declining ridership on the future Purple Line, further delaying construction of the light-rail project in the Washington suburbs and jeopardizing its chances to secure critical federal funding.

The ruling means major construction on the 16-mile line connecting Montgomery and Prince George’s counties remains on hold until a federal lawsuit filed by Purple Line opponents is resolved. Construction already has been delayed seven months because of the lawsuit.

The decision by U.S. District Judge Richard J. Leon also greatly jeopardizes the project’s chances at $900 million in federal con struction grants and threatens a $5.6 billion public-private partnership.

Maryland officials have said that if by June 1 they have “no foreseeable path” to resolve the lawsuit, they would have to suspend much or all of the rail line’s planning and design work because state money would run out. The state would then have about 60 days before it would have to cancel the project and back out of the public-private partnership — a move that could cost it more than $800 million in sunk costs and contract termination fees, Maryland Transportation Secretary Pete K. Rahn said in a recent court filing.

Maryland Gov. Larry Hogan (R) called the ruling “incredibly disappointing, but not entirely surprising.” Hogan has criticized Leon, saying the judge was delaying the case because of a conflict of interest that biased him against the light-rail project.

“The fact that it took a federal judge this long to reach the conclusion that more study is needed is completely baffling and, if allowed to stand, will cause irreparable harm to this vital project and cost the state hundreds of millions in taxpayer dollars,” Hogan said in a statement.

The statement did not address a June 1 deadline, saying only, “The state will continue to pursue any and all legal action to ensure that the Purple Line will move forward.”

Asked whether they would appeal the decision, representatives of the Maryland attorney general’s office and the Justice Department said only that they were reviewing the ruling. The Federal Transit Administration referred all questions to the Justice Department.

Purple Line Transit Partners — the team of private companies contracted to help finance the line, build it and operate it over 36 years — did not respond to emails seeking comment.

Leon said Maryland and federal transit officials must redo the Purple Line’s ridership forecasts because they hadn’t taken “the requisite ‘hard look’ at the potential impact that [Metro’s] ridership and safety issues could have on the Purple Line project.”

Leon said the government’s dismissive treatment of Metro’s potential effects was “arbitrary and capricious” and ordered the agencies to update the ridership study “as expeditiously as possible.”

Maryland officials have said reopening the study could take months, and every month of delay costs the state more than $13 million.

The decision is a major victory for Purple Line opponents, including two Chevy Chase residents and a trail advocacy group that filed the federal lawsuit in 2014 seeking to block the project on environmental grounds.

John M. Fitzgerald, an environmental lawyer and plaintiff in the case, called the ruling “a victory of substantive analysis over agency arrogance.”

He said further study might find an east-west transportation project that is “more effective, less expensive and less harmful” or perhaps persuade Maryland officials to spend more to rehabilitate Metro rather than build the Purple Line.

The ruling comes at a precarious time for the rail project because Maryland officials have said they need the federal grants to cover nearly half of the line’s $2 billion construction cost.

Congress has appropriated $325 million toward the Purple Line. However, Maryland officials can’t access that money unless Leon or another judge reinstates the project’s federal environmental approval, which Leon revoked in August.

The project also would need a federal funding agreement, and it’s unclear whether the Trump administration will sign new ones. The Purple Line was five days away from securing such an agreement when Leon’s earlier ruling made it ineligible.

The White House has said new transit projects should be “funded by the localities that use and benefit from these localized projects.” Maryland officials have said the state can’t afford to build the line without federal aid.

Ralph Bennett, president of the advocacy group Purple Line Now, said he hopes the Maryland Attorney General’s Office appeals Leon’s ruling quickly.

“Obviously, it’s a danger to the project,” Bennett said. “The judge is behaving in a way consistent with wanting to kill the project by delay.”

[How a court ruling put $900 million in federal money off limits] In his latest ruling, Leon said he still thinks the government hasn’t adequately considered the potential impact of Metro’s ridership woes.

While the Purple Line wouldn’t be part of the Metro system, more than a quarter of its passengers are expected to come from people riding Metro.

Leon acknowledged that after his earlier ruling, state and federal transit officials had submitted a previous study that found Metro would not have a significant impact on the Purple Line, even if under the most dire of five scenarios, Metro contributed no passengers to the light-rail line.

In his 12-page decision, Leon said the government hadn’t done enough to “critically assess” which of the five scenarios was most likely to occur. He called it “curious” that the government had argued that further study of Metro’s potential effects was unnecessary, especially when “providing better connections to Metrorail services” was one of the state’s three justifications for the project.

He said state and federal transit agencies also hadn’t addressed “serious questions” that the plaintiffs’ experts had raised about the Purple Line’s ridership forecasts. Those included an economist who said the Purple Line estimates were based on unrealistically rosy assumptions about how quickly Metro’s ridership would recover.

Leon said he will rule in a few weeks on a couple of dozen other legal arguments that the plaintiffs made criticizing the Purple Line’s environmental impact.

The Maryland Transit Administration relied on the Purple Line’s ridership forecasts in the project’s federal environmental review to help justify building light rail instead of a less expensive rapid-bus option, saying light rail would attract and carry more passengers.

The Purple Line is designed to better connect suburban Maryland neighborhoods inside the Capital Beltway with Metro stations and Amtrak and MARC commuter rail stations. State officials say the 21 stations will attract redevelopment, particularly in economically struggling older communities in Prince George’s County. The line also is designed to provide faster, more reliable transit service than buses.

Metro became an issue in the lawsuit last spring, when Purple Line opponents argued that the project’s environmental approval wasn’t valid because the ridership projections in it presumed a “robust” Metro system.

Metro’s ridership has fallen 12 percent since 2010, with 100,000 fewer trips daily, and the aging system continues to suffer from chronic breakdowns and safety lapses. The system’s ongoing SafeTrack maintenance program has caused ridership to further decline.

In court filings, the FTA argued that Metro’s safety and reliability problems are being addressed through SafeTrack and that ridership will rebound after the program is completed, particularly as the region continues to grow.

Even under the “most extreme” scenario — removing Metro completely from the Purple Line ridership analysis — the Purple Line would still have about 50,000 weekday trips in 2040, making it “one of the most robust light-rail systems funded by the FTA in recent years,” the FTA said.

The FTA also said that any change in ridership wouldn’t affect the light-rail line’s environmental impact because it wouldn’t change the project’s construction footprint.

The plaintiffs — Fitzgerald, Chevy Chase resident Christine Real de Azua, and the Friends of the Capital Crescent Trail — said the FTA hadn’t considered the number of Metro riders who have lost faith in the system or that Metro’s “ongoing extreme problems with safety, service and reliability are continuing unabated with no end in sight.”