CNN reports, “During the Clinton email server investigation, investigators and prosecutors debated whether to issue subpoenas to Clinton’s aides, officials say. Leaders at the FBI and at the Justice Department thought it would be faster to come to voluntary agreements with aides. Subpoenas could cause delays, particularly if litigation is necessary, officials said. And the FBI and Justice Department wanted to try to complete the probe and get out of the way of the 2016 election.”
Presumably this meant it was agreed not to get Justice Department approval to empanel a grand jury, because an FBI investigation cannot issue subpoenas without the legal authority of a grand jury. (CNN, 11/2/2016)
Two days earlier, Senator Charles Grassley (R) sent FBI Director James Comey a letter asking for an official answer regarding this issue. Comey hinted in September 2016 that he didn’t seek a grand jury in the interest of quickly concluding the investigation.