DENVER -- Thousands of ballots that went missing were never prepared because of a technical problem, the Denver Clerk and Recorder said Saturday.

More than 11,000 ballots went missing when the vendor in charge of printing the ballots, Sequoia Voting Systems, reported delivering 21,450 ballots to a Denver mail processing facility on Oct. 16, but the U.S. Postal Service said they only received 10,364 ballots that day.

Stephanie O'Malley, Denver's clerk and recorder, 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," the statement said. "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."

"We take this matter very seriously and want Denver voters to know that no one will be disenfranchised by this vendor error," the statement said.

"Sequoia made an unfortunate mistake with a portion of Denver's absentee ballots that we produced and mailed," company spokeswoman Michelle Shafer told the Rocky Mountain News Saturday. "We are extremely regretful of this error and take full responsibility for it. We are working diligently this weekend on this matter."

Elections officials said the ballots are being prepared over the weekend and will arrive at the Postal Service on Oct. 27.

The deadline for mail ballot applications is Tuesday.

The USPS said voters should mail their ballots by Friday Oct. 31 to make the Election Day deadline. Voters can also drop off their mail ballot at any early voting site or any polling place on Election Day.

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