WASHINGTON (AP) — The shifting explanations for why President Donald Trump fired national security adviser Michael Flynn have revived questions about whether the president may have obstructed an ongoing investigation of potential contacts between his campaign and Russia.

Pressure on the administration has mounted since Flynn last week pleaded guilty to lying to the FBI about his conversations with the Russian ambassador, with prosecutors revealing that he is now cooperating with special counsel Robert Mueller's investigation. And a muddled White House response, including a problematic presiden...