By JONATHAN D. SALANT WASHINGTON (AP) - The nation'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 cost. 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 $4.7 billion is part of President Bush's 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." Others in the airport management field want to be able to use alternatives to explosive detection machines. George Doughty, executive director of the Lehigh-Northampton Airport Authority, said smaller airports should be allowed to search bags by hand. The authority runs Lehigh Valley International Airport in Allentown, Pa., which handles 1 million passengers a year. "The law specifically states EDS machines," Doughty said. "If you can accomplish the same mission with another technique, I'm sure Congress would be happy with that." 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. Also to be decided is where the explosive detection machines should go. Transportation Secretary Norman Y. Mineta and many airport executives say they want them in baggage handling areas rather than in airport lobbies. "It's going to be difficult to meet any kind of customer service requirements if these machines are in front of the ticket counters," said Gina Marie Lindsay, managing director of Seattle-Tacoma International Airport, which handles 27 million passengers a year. Failure to install machines currently available before the deadline might have a positive effect, some officials said, insofar as the government might over the next year be able to help them find more modern equipment. "I don't want them to spend billions of dollars on equipment that may be outdated in a year or two," said House aviation subcommittee chairman John Mica, R-Fla. "They're making progress to acquire equipment and meet the requirements Congress has set up but we've got to do a much better job of moving forward for next generation technologies." --- On the Net: Transportation Security Administration: http://www.tsa.dot.gov Airport trade group: http://airportnet.org