According to the Journal's report, the FBI came into possession of audio showing a public-corruption suspect talk about several deals Bill and Hillary Clinton secretly brokered, presumably in connection to Hillary's private email server. Despite the clear relevance to the bureau's investigation into Clinton's emails, the Department of Justice ordered agents to not pursue the matter further:
“FBI investigators believed the audio was at least worth investigating and they grew increasingly frustrated as they ran into roadblocks erected by corruption prosecutors and even some FBI executives, sources told The Wall Street Journal. The prosecutors reportedly felt the case wasn’t worth bringing to a grand jury because the individual in the secret recording wasn’t working inside the Clinton Foundation.
FBI Deputy Director Andrew McCabe and U.S. Attorney for the Eastern District of New York, Robert Capers, were both at the center of the internal dispute, per the report:
Amid the internal finger-pointing on the Clinton Foundation matter, some have blamed the FBI’s No. 2 official, deputy director Andrew McCabe, claiming he sought to stop agents from pursuing the case this summer. His defenders deny that, and say it was the Justice Department that kept pushing back on the investigation.”
Source: Independent Journal Review