About 18,000 Denver voters who requested mail ballots still haven't received them, but officials say the missing papers have been found and should be delivered by Wednesday.

The Denver Elections Division said Monday that Sequoia Voting Systems failed to ship the ballots as promised. After complaints from voters, election officials determined over the weekend the ballots hadn't been shipped.

Sequoia, which has a $742,000 contract to print and ship Denver's mail-in ballots, has accepted blame for the missing ones but hasn't given a detailed explanation.

The Postal Service recommends mailing ballots no later than Friday. They can also be dropped off in person on Election Day.

Denver officials say Sequoia has printed and shipped about 190,000 mail ballots to Denver voters. Tuesday is the last day to request one.