But there have been concerns that the devices have had difficulty identifying liquid explosives. Officials maintain the machines are effective and none of the airports -- including Detroit Metropolitan Airport -- have plans to stop using them.
"Before we deploy the next equipment, we are working out some operational maintenance issues," said Lara Uselding, a public affairs manager for the Transportation Security Administration. "With the next generation of equipment that will be rolled out, we're looking at the information we gathered from the 95 pieces of bomb detection equipment we have deployed around the country."
She said delivery of 295 of the $160,000 machines will be delayed indefinitely. "It's an opportunity for us to make improvements on the next generation of equipment before it is sent out."
Officials declined to discuss specific problems with machines in use. Detroit Metropolitan Airport has three of the security devices and was expecting a fourth.
With the portal machines, a passenger walks into a booth-like device in the screening area. A series of jet nozzles shoot puffs of air on the passenger and the air is analyzed for explosives.
Because only three were installed at Metro Airport, where about 20 million passengers pass through security each year, TSA selects certain passengers for the high-tech screening. Everyone passes through regular screening with metal detectors and pat downs.
When the devices were installed nine months ago, Bob Ball, Metro Airport's federal security director, said they would shorten screening time and allow selected passengers to pass through both the high-tech and regular security measures without removing their shoes. The shoe policy changed after the foiled terrorist plots on airlines between London and the United States last month.
Ball said he hasn't detected a problem with Detroit's machines and uses them every day. The airport hasn't received any alerts from TSA headquarters warning of a problem.
"Headquarters is doing sensitivity and reliable tests on the machines, but we don't have any results yet," Ball said. "We don't have any local issues that we have detected yet."
Ball said one machine required more maintenance, but that issue was resolved by adopting a policy to change air filters on all the machines once a day.
"I'd like to have enough money to put one of these devices at every door entering that terminal. But that's never going to happen," he said.