DENVER -- Denver elections officials are trying to determine the whereabouts of more than 11,000 mail ballots, the Rocky Mountain News reported Friday.

Sequoia Voting Systems reported delivering 21,450 ballots to a Denver mail processing facility on Oct. 16, but the U.S. Postal Service said Friday that the facility received 10,364 ballots that day, the newspaper reported. The Postal Service said it delivered the ballots to the homes within a couple of days.

The discrepancy prompted Denver Election Director Michael Scarpello to ask Sequoia for paperwork proving the ballots were shipped, the newspaper reported.

"Certainly we're concerned and we're wanting to get to the bottom of it," Scarpello said. "I don't think we've seen overwhelming evidence to prove one theory or the other.

"My advice to people would be to hang tight for another couple of days."

Sequoia Voting Systems, based in Porterville, Calif., was working to find the paperwork from its mail house to show that the ballots were delivered to the post office, company spokeswoman Michelle Shafer said.

"We will continue to work with all parties concerned to get to the bottom of this situation and rectify it as necessary," she said.

Postal Service spokesman Al DeSarro said his office has delivered all ballots it received.

Scarpello said he is certain that his office sent the correct number of voter files to the printer. He said the problem appears to be either at the printer or at the post office.

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

Voters have until Oct. 28 to request mail-in ballots.