Chertoff said an individual or small group planning to do harm is difficult to detect.
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However, carbon-14 is only found in trace amounts, which can make it difficult to detect.
Carbon monoxide (CO) is difficult to detect because it is colourless and has no taste or smell.
They take a low-and-slow approach that often makes them difficult to detect, giving them a high likelihood of success.
Unless you know what to look for, the differences in weight, workmanship and appearance are difficult to detect between original and fake.
Having no propellers and no engine noise, they are difficult to detect.
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The danger is made worse because the ways in which Google is alleged to have tilted the playing field are difficult to detect.
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Wily suspects may try to raise their baseline readings artificially (by doing mental arithmetic, for example), making later fibs more difficult to detect.
Pollock's use of disguised and obscured representation, although later difficult to detect, persisted into his abstract masterpieces of the late '40s and early '50s.
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Electric and hybrid vehicles do not rely on traditional gas or diesel-powered engines at low speeds, making them much quieter and their approach difficult to detect.
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That makes them difficult to detect, and means that neutrino telescopes have to be built underground to stop the detectors registering other forms of radiation by mistake.
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In addition, the inside of the trunk lid was partially spray-painted to create a light trap, making it more difficult to detect when the lid was cracked open.
Dr Burke announced 461 new candidate planets, a substantial fraction of which were Earth-sized or not much larger - planets that have until now been particularly difficult to detect.
It turns out to be very difficult to detect lies reliably, but Paul did improve the odds a little with training that he developed for the CIA and the FBI.
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Few maps are made of where the mines are laid, and since the home-made varieties contain very little metal, they are extremely difficult to detect and so sensitive that even sniffer dogs can detonate them.
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Not only does the drone likely use advanced coatings and materials to make it difficult to detect by radar, but the drone also likely has the U.S.'s most-cutting-edge sensor technology, including sophisticated cameras and listening devices.
In a statement issued to CNN in 2006, the USDA said it does not support 100% testing because the disease is difficult to detect in young cattle, the primary source of beef in the United States.
Which means that it is very difficult to detect a rational motive for the taking of real bets matched by offsetting fictional ones, because it is very difficult to see how any individual could have profited by such a stratagem.
Dr Jel Coward, a GP in Tywyn, north Wales, and an expert on wilderness medicine, said people suffering from extreme hypothermia often gave the impression of being dead, particularly as it can be difficult to detect breathing or a pulse.
An electron microscope would be unreliable, particularly for someone infected a long time, because the number of cells harboring virus genes would be diluted over time and difficult to detect, said McGowan, the New York and New Jersey chairman of the American Academy of HIV Medicine.
Thanks to a combination of "stealthy" characteristics that make it very difficult to detect and target, the ability to operate for sustained periods at supersonic speeds and its extraordinary agility, the Raptor seems likely to secure for years to come something Americans have taken for granted in every conflict since World War II: air superiority essential to victory on the ground.
For example, most of the energy from supernovae is thought to be carried away by neutrinos -- the lightweight, difficult-to-detect particles that are so important to understanding the fundamental laws of nature.
"Offences of this nature are easy to commit but very difficult and very expensive to detect, " he said.
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Many viruses are harmless in their regular hosts (and therefore difficult or impossible to detect) but devastating if they switch to a new one.
Marc Goodman, a global security expert and the track chair for Policy, Law and Ethics at Singularity University, explains just how difficult it is to detect these types of attacks.
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