As President Vladimir V. Putin of Russia has pursued enemies abroad, his intelligence operatives now appear prepared to cross a line that they previously avoided: trying to kill a valuable informant for the U.S. government on American soil.
The clandestine operation, seeking to eliminate a C.I.A. informant in Miami who had been a high-ranking Russian intelligence official more than a decade earlier, represented a brazen expansion of Mr. Putin’s campaign of targeted assassinations. It also…