The White House did not invite the most senior American official charged with preventing terrorist attacks — the F.B.I. director, James B. Comey — to the three-day conference this week on countering violent extremism in the United States and abroad because the administration did not want the event too focused on law enforcement issues, according to senior American officials. But Mr. Comey’s Russian counterpart — Aleksandr V. Bortnikov, the director of the Russian Federal Security Service, the post-Soviet K.G.B. — was at the meeting, even though international human rights groups have repeatedly accused the Russian security service of unjustly detaining...