In those meetings and others, the special counsel investigating Russian interference in the 2016 presidential election strictly limits the number of outsiders who can attend. Militant about leaks, the former FBI director swears participants to a secrecy that they have honored to a remarkable degree. Reporters have long considered him among Washington’s toughest nuts to crack: “You’d be embarrassed to ask Bob Mueller for a leak,” said the veteran journalist Steven Brill, who has written extensively about media coverage of special counsels. “It’d be like asking him to watch a porn movie with you.”