Colorado's voter registration system experienced a 29-minute outage in some parts of the state, according to Lynn Bartels, spokeswoman for the Colorado secretary of state.
As of 3:16 p.m. local time, the system had been restored.
Clerks were unable to process mail-in ballots during the outage, Bartels wrote on Twitter. In-person voters had to use provisional ballots.
The Denver Elections Division confirmed the outage in Denver and several other parts of the state to Mic in a phone interview.
According to polling site FiveThirtyEight, Colorado has a 77.5% chance of giving Hillary Clinton its 9 electoral votes.