While the Pentagon has already begun purchasing the planes , questions remain about the design, manufacturing and time frame for production, the Government Accountability Office, Congress' investigative arm, reported.

As a result, there is a "very significant risk" that the program, already saddled with a ballooning price and delays, could experience further cost increases and more missed deadlines, the GAO said. More problems could arise as the fighter is flight-tested.

Despite the hurdles for the fighter, Rep. Rob Bishop, R-Utah, says the Pentagon should remain committed to the program so the military has the technology "critical to our entire national defense."

"Whenever the next level of military hardware or technology is developed, there are going to be challenges. That doesn't mean we give up," Bishop said. "The F-35 represents the next level, and it is what will help us maintain air superiority - something we've taken for granted for probably too long."

Designed as the centerpiece of the Pentagon's future air strike capability, the F-35 Lightning II has been put on an aggressive development and acquisition schedule. Along the way, the cost has increased dramatically - from a projected $81 million per plane in 2001 to $112 million as of 2005.

The cost of the program has risen $19.8 billion since the GAO last reviewed the project one year ago.

In October, the Air Force announced that the 388th Fighter Wing at Hill Air Force Base would be one of the units selected as the home for operational squadrons of the F-35, pending completion of environmental analyses.

The GAO recommended that the Pentagon limit its future production to no more than 24 planes a year to reduce risk, but the Pentagon rejected that suggestion, saying a major design flaw is unlikely and the planes would cost more if the procurement is extended.

The Bush administration had planned to ask Congress to provide funds for two planes in the emergency war supplemental budget, but it later removed the money from the request, using it instead to fund a surge of troops in Iraq. It has also reduced the number of fighters in the 2008 budget.

Since the program began, the projected number of fighters the department plans to purchase has been scaled back from 2,998 to 2,458.

The fighter program is expected to cost, on average, $12.6 billion a year over the next 20 years. It will have to compete with other big-ticket defense programs, such as the missile defense system.

"Escalating costs are a by-product of delays in funding and cuts in the number of planes to be built," Bishop said. "But the bottom line is they need to be funded and built."

gehrke@sltrib.com