That drone did not carry any weapons or explosives, Israeli officials said at the time.

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As just one example, emerging technology to find weapons or explosives hidden beneath clothes is devilishly challenging.

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Although a mass of data was discovered on computer hard drives, no weapons or explosives were found.

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Gerry O'Hara says this is simply untrue and that the Fianna had no access to weapons or explosives.

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Police had no idea whether he had weapons or explosives with him, so they repeatedly told him to stand up and lift his shirt to show he wasn't wearing a device, Deveau said .

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Testifying to Congress recently, Tara O'Toole of the Johns Hopkins University School of Public Health said that anti-terrorism programmes have concentrated chiefly on the threat posed by conventional explosives or chemical weapons.

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Investigators have been asking Tsarnaev whether there are more bombs, explosives caches or weapons beyond those already found by police, and if anyone else was involved in the attacks, a source with direct knowledge of the investigation told CNN.

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Investigators have been asking Tsarnaev whether there are more bombs, explosives caches or weapons beyond those already found by police, and if anyone else was involved in the attacks, a source with first-hand knowledge of the investigation told CNN.

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