Washington ? The government is scrapping a post-Sept. 11 airport screening program because the machines did not operate as well as intended and cost too much to maintain.

The so-called “puffer” machines were deployed to airports in 2004 to screen randomly selected passengers for bombs after they cleared the standard metal detectors. The machines take 17 seconds to check a passenger and can analyze particles as small as one-billionth of a gram.

But they would also break down when exposed to dirt or humidity, the Transportation Security Administration said in a statement released Thursday. Since 2005, maintenance of the machines cost taxpayers more than $6 million.

The government has spent $42.3 billion on aviation security since the Sept. 11, 2001, terror attacks, according to TSA.