US stocks had been comfortably in positive territory prior to news that Federal Bureau of Investigation head James Comey informed lawmakers that he was restarting the probe.
Comey in June sharply criticised Clinton's handling of classified information as Secretary of State on a private email server, but did not recommend charges be brought against her.
Near 17.30 GMT, the S&P 500 was down 0.3 per cent at 2,127.70 after hitting a session high of 2,140.72 shortly before the news broke.