Washington ? U.S. airports face at least $2 billion in construction costs to make room for machines to detect explosives. Officials say they don’t know how many machines they need, where they should be installed or who will pick up the tab.

Renovations for the most part have yet to begin and it’s unlikely airports will meet an end-of-year deadline for having all the equipment in place.

“As we sit here, we don’t know what to build or where to build it,” said Jim Wilding, president of the Washington Metropolitan Airports Authority, which runs Washington Dulles and Reagan National airports and handles 35 million passengers annually.

“There’s just a whole host of very complicated, very expensive decisions that need to be made,” he said. “You wish they could have been made last week or last month, but they just haven’t been.”

The new aviation security law requires explosive detection systems at all 429 commercial airports by Dec. 31 to inspect checked baggage. But the airports have to make room for the machines.

Transportation Department Inspector General Kenneth Mead estimated the cost of renovating the airports at more than $2 billion, to accommodate more than 2,000 machines.

The government will pay installment costs of $175,000 per machine, said Transportation Security Administration spokesman Jonathan Thompson. The money for the machines is part of the $4.7 billion President Bush included in a supplemental budget proposal.

Because airport executives said there isn’t enough time to complete the renovations by year’s end, they will need to use other technologies as well, such as handheld equipment to detect traces of explosives, in addition to the explosive detection machines.

“We tend to think in terms of a couple of years to build things rather than a couple of months,” Wilding said. “It’s increasingly likely that a combination of technologies is going to be necessary rather than going directly to a permanent solution.”

At Salt Lake City Airport, which handles 19 million passengers a year, checked bags are first inspected with the handheld equipment, for instance. They are sent through explosive detection machines only if there is something suspicious.

“A system such as ours is the only way that all airports can comply with the law,” said Tim Campbell, the airport’s executive director. “There’s no way the manufacturers can even manufacture enough machines, let alone have the airports in a position to install and retrofit their terminals.”

Sen. Richard Durbin, a member of the Senate Appropriations transportation subcommittee, acknowledged there may not be enough machines in place by Dec. 31. “As long as we’re making a good-faith effort  and I think we are  I’m not going to be critical,” said Durbin, D-Ill.