The current samplers are vacuum-powered collection devices, about the size of an office printer, that pull air through filters that trap any airborne materials. In more than 30 cities each day, technicians collect the filters and deliver them to state or local health labs for genetic analysis. Lab personnel look for DNA matches with at least half a dozen targeted pathogens.
The new, larger units would be automated labs in a box. Samples could be analyzed far more quickly and with no need for manual collection.
Buying and operating the new technology, known as Generation 3, would cost about $3.1 billion over the next five years, on top of the roughly $1 billion that BioWatch already has cost taxpayers. The Obama administration is weighing whether to award a multiyear contract.
Generation 3 "is imperative to saving thousands of lives," Dr. Alexander Garza, Homeland Security's chief medical officer, told a House subcommittee on March 29.
But field and lab tests of automated units have raised doubts about their effectiveness. A prototype installed in the New York subway system in 2007 and 2008 produced multiple false readings, according to interviews with scientists. Field tests last year in Chicago found that a second prototype could not operate independently for more than a week at a time.
Most worrisome, testing at the Pacific Northwest National Laboratory in Washington state and at the Army's Dugway Proving Ground in Utah found that Generation 3 units could detect a biological agent only if exposed to extremely high concentrations: hundreds of thousands of organisms per cubic meter of air over a six-hour period.
Most of the pathogens targeted by BioWatch, scientists said, can cause sickness or death at much lower levels.
A confidential Homeland Security analysis prepared in January said these "failures were so significant" that the department had proposed that Northrop Grumman Corp., the leading competitor for the Generation 3 contract, make "major engineering modifications."
A spokesman for the department, Peter Boogaard, defended the performance of BioWatch. Responding to written questions, he said the department "takes all precautions necessary to minimize the occurrence of both false positive and false negative results."
"Rigorous testing and evaluation" will guide the department's decisions about whether to buy the Generation 3 technology, he said.
Representatives of Northrop Grumman said in interviews that some test results had prompted efforts to improve the automated units' sensitivity and overall performance.
"We had an issue that affected the consistency of the performance of the system," said Dave Tilles, the company's project director. "We resolved it. We fixed it.... We feel like we're ready for the next phase of the program."
In congressional testimony, officials responsible for BioWatch in both the Bush and Obama administrations have made only fleeting references to the system's documented failures.
"BioWatch, as you know, has been an enormous success story," Jay M. Cohen, a Homeland Security undersecretary, told a House subcommittee in 2007.
In June 2009, Homeland Security's then-chief medical officer, Dr. Jon Krohmer, told a House panel: "Without these detectors, the nation has no ability to detect biological attacks until individuals start to show clinical symptoms." Without BioWatch, "needless deaths" could result, he said.
Garza, the current chief medical officer, was asked during his March 29 testimony whether Generation 3 was on track. "My professional opinion is, it's right where it needs to be," he said.
After hearing such assurances, bipartisan majorities of Congress have unfailingly supported additional spending for BioWatch.
The problems inherent in what would become BioWatch appeared early.
In February 2002, scientists and technicians from Lawrence Livermore National Laboratory deployed a prototype in and around Salt Lake City in preparation for the Winter Olympics. The scientists were aware that false alarms could "cause immense disruptions and panic" and were determined to prevent them, they later wrote in the lab's quarterly magazine.
Sixteen air samplers were positioned at Olympic venues, as well as in downtown Salt Lake City and at the airport. About 5:30 p.m. on Feb. 12, a sample from the airport's C concourse tested positive for anthrax.
Utah Gov. Mike Leavitt was at an Olympic figure skating competition when the state's public safety director, Bob Flowers, called with the news.
"He told me that they had a positive lead on anthrax at the airport," Leavitt recalled. "I asked if they'd retested it. He said they had — not just once, but four times. And each time it tested positive."
The Olympics marked the first major international gathering since the Sept. 11, 2001, airliner hijackings and the deadly anthrax mailings that fall.
"It didn't take a lot of imagination to say, 'This could be the real thing,'" Leavitt said.