More than 150,000 ballots were caught in U.S. Postal Service processing facilities and not delivered by Election Day, agency data shows, including more than 12,000 in five of the states that have yet to be called for either President Trump or Democratic challenger, Joe Biden.

Despite assurances from Postal Service leaders that agency officials were conducting daily sweeps for misplaced ballots, the mail service acknowledged in a court filing Thursday that thousands of ballots had not been processed in time, and that more ballots were processed Wednesday than on Election Day.

The number of mailed ballots the Postal Service did not deliver by Election Day is expected to grow as more data is released in the coming days. Some election experts worry such delays could run up against even more generous ballot acceptance windows that some states have granted.

In several swing states, late ballots will still be counted as long as they were postmarked by Election Day and received by Friday, according to state law.