Leaders of Butler County, Pennsylvania, said on Thursday that they've "pulled out all the stops" in an effort to get ballots to "thousands" of voters who have reported not yet receiving them just days before the presidential election, even as it's still not clear what happened to many of the approximately 40,000 ballots they say they mailed out.

Of those 40,000 or so sent ballots, about 21,300 have already been returned. But officials say they are inundated with calls from other voters who have yet to see their ballots well past the time when the U.S. Postal Service recommends mailing ballots to ensure elections boards receive them before Nov. 3.

The time-sensitive effort involves approximately 40 people, officials said, including a triage call center, deputy sheriffs delivering ballots to those who can't go vote in person, a team checking in the ballots that are getting back to the county, individuals in the lobby of the elections bureau taking ballots that are being hand-delivered and senior county staff pitching in as well. 

"We've pulled out all the stops," Elections Director Aaron Sheasley told Fox News Thursday. "We are 100% focused on making sure that everybody gets their ballots and that they are able to get them back to us in time to be counted for the election."

SUPREME COURT COULD STILL OVERTURN PENNSYLVANIA MAIL-IN BALLOT DEADLINE EXTENSION AFTER ELECTION

Butler County Board of Commissioners Chair Leslie Osche added: "We have our deputy sheriffs... probably two at a time every several hours... We have our entire commissioner's office staff returning phone calls and taking calls. The Veterans Bureau staff is taking calls. The D.A. is preparing for the election. The sheriff is taking calls. I mean, it really is, it's pretty much the entire building at this point."

Osche said the Butler County Bureau of Elections has taken "thousands of calls and emails from voters saying they did not receive their mail-in ballot."

The Butler Eagle first reported that Sheasley told the commissioners during a Wednesday meeting that USPS officials said the service was investigating reports the county has received about voters not getting their ballots. 

Osche said Thursday that USPS "is maintaining daily contact with [the] Elections Bureau and [is] aware of the situation."

USPS said in a statement to Fox News that it's been in contact with Butler County but that it isn't aware of any specific issues in the county. USPS also said that it is not aware of a "federal investigation" into allegedly lost ballots in Butler County.

'The most important thing to do right now is getting everyone ballots that may have fallen into this void and to... get them as fast as they can, and then to get them back to us so they can count'

— Butler County Elections Director Aaron Sheasley

"Regarding mail sorting and delivery in Butler County, the Postal Service is unaware of any significant delays or issues and is in regular contact with the Board of Election as we work to locate and deliver ballots as they are presented to us," a USPS spokesperson told Fox News. 

"With a record number of people across the country voting by mail, the U.S. Postal Service’s number one priority between now and the November election is the secure, timely delivery of the nation’s Election Mail," the statement also said. 

"We don't know the number of people that have been impacted by this. That's something we're looking into," Sheasley told Fox News Thursday. "But... we're focused more at this point on making sure people get their ballots and are able to exercise their right to vote... We're a couple days out now and that's the most important thing."

Sheasley added that the county would do more "digging" into the root cause of the problem after the election. 

"We are reaching out to both Butler County and the USPS to obtain more information," a spokesperson for the Pennsylvania Department of State said Thursday. The spokesperson also added that voters who did not get their mail ballots yet should go to the elections bureau for a replacement ballot or simply go to their polling place on Election Day and vote via provisional ballot. 

Butler County officials said the state department reached out to them after noticing a disproportionately low number of mail-in ballots being returned in their county and also reached out to the postal service on its own. 

The Butler County Bureau of Elections tells voters it is preferred that they vote in-person at their polling places on Election Day if they haven't gotten their ballots. There are 89 polling places that will be open in Butler County and Sheasley said resources are being directed to ensure those are run well.

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Butler County is not the first Pennsylvania county to have issues with its mail ballots. Allegheny County, which is the county where Pittsburgh is located, said officials sent almost 29,000 incorrect ballots to voters earlier this month. The county said that it was quickly correcting the error and would send correct ballots. 

Allegheny County said that it was looking for any incorrect ballots that were mailed and would set them aside, and that the county election board would ensure only one ballot per voter is counted. 

Pennsylvania is one of many U.S. states that are leaning more heavily on mail-in and early voting amid the coronavirus pandemic. Pennsylvania is a no-excuse absentee state, meaning that voters were able to request mail ballots without giving a reason. It is not sending ballots to all voters -- in a system that has been termed universal mail-in voting -- the way some states are doing for the 2020 election. 

Butler County "is doing everything imaginable and then some," Sheasley said of the county's efforts to ensure people are able to exercise their right to vote despite the confusion just days before Election Day. 

"The most important thing to do right now is getting everyone ballots that may have fallen into this void and to... get them as fast as they can, and then to get them back to us so they can count," Sheasley said. 

The Associated Press contributed to this report.