By all means, perform your investigation, but to announce it immediately before what's arguably one of the most important elections in modern US history? If I were a paranoid man, I'd say that this looks like an attempt by an unelected official to prejudice voters.
Under such circumstances, the exposure of a search for a smoking gun is suspect.
Waiting until after seems wrong, and I say this as someone who as it stands is voting for HRC.
I honestly don't view this as influence, and more covering his ass, which has me nervous for HRC.
Unfortunately, all we do know is that they might have found something that might be important enough to warrant an investigation that might result in an indictment recommendation, &c, &c. With this in mind, it's more than a little strange.
Unless I've misunderstood Comey's powers of discretion over FBI investigations (which is entirely possible), he has little to no need to "cover his ass."
Disclosure of a new investigation (or a new branch of the current investigation, as his letter apparently actually says) is prejudicial in all the same ways that perp walks are.