Local gunmen in eastern Ukraine ousted the first outsiders seeking to safeguard the crash site of Malaysia Airlines Flight 17, even as authorities in Kiev began marshaling a wide-ranging team of international aviation and law-enforcement investigators to probe the downing of the jetliner.

World leaders, including U.S. President Barack Obama, joined a chorus calling for a transparent, credible probe of the crash. Global aviation experts—typically deployed to crash sites involving fatal, high-profile accidents of big jetliners—are...