US authorities added the name of the Boston bombing suspect Tamerlan Tsarnaev to a terrorist database 18 months before the explosions, but did not follow up their inquiry.
Officials say the CIA made the request to add Tamerlan's name after the Russian government contacted them with concerns he had become a follower of radical Islam.
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Labour have lost the Copeland by-election, a seat they had held since 1983, as the Conservatives edged the vote by 2,000 votes.
Labour have held onto the crucial seat of Stoke-on-Trent, brushing aside UKIP leader Paul Nuttall in the process.
A highly toxic chemical was used in the death of North Korean leader Kim Jong-un's half brother last week, tests have shown.