U.S. Postal Service didn’t deliver more than 150,000 ballots by Election Day.

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 than12,000 in five of the states that have yet to be called for President Donald Trump or 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 tens of thousands of ballots hadn’t been processed in time, and that more ballots were processed Wednesday than on Election Day.

The number of mailed ballots the Postal Service didn’t 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 some of the more generous ballot acceptance windows some states have granted.

In several swing states, late ballots still will be counted as long as they were postmarked by Election Day and received by Friday, according to state law. They include Nevada, where 4,518 ballots arrived after Election Day, as well as North Carolina (2,958) and Pennsylvania, (3,439).

But in other states — such as Arizona, where 864 ballots were delayed, and Georgia (853) — votes that didn’t reach election officials by Nov. 3 will be disqualified.

Because the counts aren’t finished in those states, it’s unclear whether undelivered ballots would have made a difference in deciding the presidential election. But the delivery failures highlight the risks in relying on the mail service to deliver ballots close to Election Day.

The Postal Service had warned voters not to mail ballots within one week of the election.

The Biden campaign changed its messaging to encourage voters to use drop boxes or vote in person and to avoid mailing their ballots within 10 days of the election.

The Trump campaign’s messaging on the topic was mixed: The president claimed mail-in voting was susceptible to fraud, though he and first lady Melania Trump voted by mail themselves in Florida, and the campaign later courted supporters to vote by mail after seeing growing Democratic advantages.

Under Postmaster General Louis DeJoy, a major Trump financier who took over the agency in June, first-class mail delivery rates have declined steadily, especially in urban areas that are bastions of Democratic voters.

DeJoy said in public statements that facilitating an election in which a record 198 million Americans were eligible to vote by mail was his “sacred duty” and that the agency was up to the task. But the Postal Service’s summer struggles bled into election season almost immediately as Americans began voting in October.

On Wednesday, the Postal Service processed 94.5 percent of ballots on time, an improvement over recent days, but below the 97 percent rate that postal and voting experts expect. Based on the agency’s one- to three-day ballot-processing window, voters would have mailed those ballots Sunday or Monday.

For the remaining 5.5 percent, or more than 8,000, of the ballots that took longer to process, they would have been mailed between Thursday and Saturday.

In the nine postal districts spanning five states — Arizona, Nevada, Pennsylvania, North Carolina and Georgia — that have yet to decide the presidential race, the on-time rate was 84.6 percent. That means roughly 15 of every 100 ballots in processing plants were not sorted — or delivered — in time.

A Postal Service representative didn’t immediately respond Thursday to a request for comment.