A rail yard critical to the Silver Line is behind schedule, and project officials are disputing the cause of the delay and have yet to agree on a faster timeline.

Metro officials warned more than three months ago that a delay for the rail yard could mean a late opening for the first phase of the Silver Line, from East Falls Church to Reston, scheduled for late this year.

The Metropolitan Washington Airports Authority is overseeing the line's construction and is due to turn it over to Metro in September for testing. But a rail yard at West Falls Church that Metro officials say they need in order to start full service isn't expected to be done until Jan. 17, 2014, according to the airports authority's latest progress report.

Airports authority officials said its contractor and subcontractors still have not given acceptable reasons for the delay or come up with a way to fix it.

"The [airports] project team continues to identify apparent and implied shortfalls in [the contractor]'s schedule," the report says. "[Contractor]-caused delays ... are causing the entire project to become critical."

Construction giants Bechtel and URS are building the new rail line. A spokeswoman for the companies said they prefer not to comment on the specifics of the report.

"The project is on schedule and the entire team is focused on keeping it that way," said Leslie Pereira, a spokeswoman for the companies.

Metro said it can start operating the rail line with part of the rail yard as long as it has extra storage tracks, a maintenance building and a sound wall.

Airports authority officials said they are working on a schedule to make sure Metro gets those things so it can open the rail line on time.

"We're drawing up a schedule right now to determine what needs to be done in the next three months," said project director Charles Carnaggio. "We don't believe we're in danger of impacting the schedule. We meet with [Metro] every Friday, and they seem to be onboard with that."

Metro officials said they expect the airports authority to pay for any delays.

"It may be possible to take extraordinary steps for a short period to offset the shop not being ready," Metro spokeswoman Caroline Lukas said in an email. "Any cost associated with the yard not being ready on Day 1 would be borne by MWAA."