DENVER -- More than 18,000 mail-in ballots were scheduled to be delivered to waiting Denver voters Tuesday and Wednesday.

The ballots are coming from Sequoia Voting Systems in California, which failed to send them out earlier this month and to notify Denver election officials that they had not been mailed.

Denver Clerk and Recorder Stephanie O'Malley said, "We are very sure at this point that every unsent ballot has been accounted for."

The ballots were discovered missing last weekend, but Sequoia originally said it mailed the ballots.

O'Malley led an investigation by the Denver Elections Division that discovered Sequoia Voting Systems didn't prepare the mail-in ballots because of a technical problem with a data file.

"Denver processed mail ballot applications in a timely manner and did not incur any problems transmitting its data file to Sequoia," O'Malley said in a statement. "Sequoia did not report any problems with the file to the Elections Division nor did they inform the Elections Division that they delivered only approximately half the requested ballots."

After rechecking numbers with Sequoia, it was learned that the total number of unsent ballots was 18,055.

If Denver voters who requested mail-in ballots don't get them in Wednesday's mail, they should called the Denver Elections Division for a replacement ballot. The number to call after 8 p.m. Wednesday is 720-913-8683.

The completed ballots must be received by Nov. 4. They can also dropped off at 13 early-voting locations or at any polling place on Nov. 4.

A record 1.5 million voters statewide have requested to vote by mail, and more than 180,000 Denver voters have requested mail ballots.