A former Twitter exec turned whistleblower says the firm poses a threat to US national security. CNN Correspondent Donie O'Sullivan has our exclusive report. Plus: The State Department is advising Americans to leave Ukraine immediately, fearing Russia will ramp up attacks ahead of Ukraine's independence day tomorrow. CNN Senior International Correspondent David McKenzie reports live from Kyiv. And: What's next for Wall Street following yesterday's across-the-board selloff?

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