The nonprofit Washington Airports Task Force on Tuesday recommended trimming up to $780 million from the ballooning cost of the second phase of Metrorail’s extension to Loudoun County.

Leo Schefer, president of the Dulles-based group, called funding of the estimated $3.5 billion project “the most serious crisis” since the White House’s decision to close Reagan National Airport for several weeks after 9/11.

“We’re very concerned at what is happening with phase two of Dulles rail,” Schefer said.

The task force’s recommendations follow the Metropolitan Washington Airports Authority’s decision last month to build an underground Metro station at Dulles International Airport, despite criticism from local funding partners who favor a less-expensive aboveground station.

But Schefer said that cost reductions for the entire rail project should be examined instead.

“The focus tends to be on the station being above and below ground. I don’t think that’s the issue,” he said.

The task force, which is funded by both public grants and the private sector to support aviation services, recommended building the station above ground, which would save the project about $330 million.

It also suggested that the airports authority work with Loudoun and Fairfax counties to negotiate third-party agreements for parking garages at Metro stations along the extension. That strategy would save the project between $100 and $200 million, the group said.

The task force recommended building an initial, scaled-back rail yard at the airport, to save between $50 and $100 million, and proposed that an interest-free state loan or grant would reduce project costs by $100 to $150 million.

The task force warned that additional borrowing from the bond market to fund the rail extension could jeopardize the airports authority’s ability to enhance passenger service and expand Dulles airport.

The 23-mile Metrorail extension to Route 772 in Loudoun County is being built in two phases. The first phase, connecting to the existing system near East Falls Church and extending to Wiehle Avenue in Reston, is scheduled to open in 2013 with five stations. Phase 2 includes six stations, including the one at Dulles, and could open by 2016.