The California High Speed Rail Authority board unanimously approved on Thursday a $985 million bid to build the first stage of the rail line by a joint venture that includes Sylmar construction firm Tutor Perini Corp. The six-member board authorized its chief executive to sign a contract with the group, which includes Parsons Corp. of Texas and Zachry Construction of San Antonio, Texas. Tutor has come under heavy scrutiny for its low-cost bid, which was eliminated from the first round of proposals after recording the lowest technical score of all bids in safety, design, engineering, scheduling and other factors. A mid-evaluation rule change allowed the joint venture to remain in the running. Its bid was the only one to come in under $1 billion. The contract is for the first 30 miles of the high-speed rail line, stretching from Fresno to Bakersfield. Work is expected to be completed on that portion in December 2017. Shares of Tutor Perini fell 27 cents, or 1.5 percent, to close at $17.35 on the New York Stock Exchange.