Officials in charge of constructing the $6 billion Silver Line tentatively selected a local firm to build the second half of the line from Reston to Loudoun County.

Capital Rail Constructors -- a team composed of Bethesda-based Clark Construction Group and Kiewit Infrastructure South Co. -- submitted the lowest bid to build the project, and after reviewing the math behind the $1.18 billion bid, officials in charge of the project announced Thursday they intend to award the contract to the group. That means that unless another contractor files a legal protest with considerable concerns, or unless Capital Rail fails to meet a few other requirements, the group will win the final contract award in May, project leaders said.

Bids for the project came in lower than expected; engineers initially estimated the work would cost between $1.4 and $1.6 billion. Since tolls from the Dulles Toll Road will pay for about 75 percent of the rail line, the lower bid could save toll payers money, officials said.

"We are very encouraged by the price submitted by Capital Rail Constructors and the potential savings it includes," said Pat Nowakowski, who is managing construction of the Silver Line for the Metropolitan Washington Airports Authority. "The winning proposal is well below our original estimates of $1.4 billion to $1.6 billion for this portion of the project, which hopefully will allow us to pass on additional savings to users of the Dulles Toll Road."

Dulles Transit Partners, a group led by construction giant Bechtel, is building the Silver Line's first phase, expected to be completed this year, which will take the line from East Falls Church to Reston via Tysons Corner. The second phase will extend the line from Reston to Loudoun County, past Washington Dulles International Airport.

Capital Rail Constructors' bid was just $14 million below the second lowest bidder, incumbent builder Bechtel. Three other firms also bid on the project.

If Capital Rail is awarded the contract without delay, work on the second phase could begin in July and be complete in 2018.

The second phase of the Silver Line was delayed for months as local, state and national officials squabbled about how to pay for it and whether it should include a union-friendly labor agreement. The airports authority came under fire in 2011 when it chose to build a more expensive, underground station at Dulles airport, despite concern from local lawmakers, and was later forced to reverse its decision.