In a surprise move, the agency has reopened an environmental study because more flights are planned.

EVERETT — The much-anticipated start of commercial air service at Paine Field, which was expected to begin this fall with completion of a passenger terminal, could be delayed by renewed Federal Aviation Administration scrutiny.

In a surprise move, the agency has begun what it calls a supplemental environmental assessment of the effect of up to 24 flights per day by three airlines. A 2012 assessment, which approved of passenger service at Everett’s county-owned airport, involved different airlines and only up to 12 flights per day.

Airline interest in Everett has intensified since. The more ambitious scope of passenger operations at Paine Field necessitated another look, an FAA spokesman said Thursday.

The agency could not say how extensive the study might be, but supplemental environmental assessments typically take six to 18 months, the FAA said. That time frame includes a public comment period.

Paine Field spokesman Scott North said commercial passenger operations “will begin when the FAA completes its regulatory process. If the FAA approves it, we expect operations to begin late in 2018.”

Everett-based Propeller Airports, under a lease agreement with Snohomish County, has invested some $40 million in developing the two-gate terminal, which is now under construction.

Propeller CEO Brett Smith said the unique circumstances of launching passenger service in Everett mean all of the involved parties are in somewhat uncharted territory — including the federal government.

“There is some ambiguity around this because this hasn’t been done in a while,” Smith said. “Commercial airline service doesn’t get added to general aviation airports very often. So the FAA, the airlines and Propeller Airports all want this done correctly.”

Alaska Airlines, United Airlines and Southwest Airlines have proposed a combined 24 flights per day to cities throughout the West.

Because the three airlines weren’t involved in the original proposal of a dozen daily flights, they can’t simply elect to fly 12 or fewer flights, the FAA said. The agency only reviews the proposals as they are submitted.

When passenger service at Paine Field was first proposed, there was vocal opposition. An organization called Save Our Communities and the city of Mukilteo were involved in a series of unsuccessful court battles to stop the terminal. Last summer, the state Supreme Court declined to hear their case, bringing the lawsuit against Propeller and airport owner Snohomish County to an end.

Propeller points out the currently planned service mostly involves smaller, quieter jets than those in the 2012 proposal.

Southwest Airlines eagerly awaits the completion of the terminal and the FAA environmental authorization, the company told The Herald. Alaska Airlines and United Airlines did not respond to requests for comment.

Herald writers Chuck Taylor and Noah Haglund contributed.

Janice Podsada: 425-339-3097; jpodsada@heraldnet.com.