© Morry Gash/AP Voters line up observing social distancing at Riverside High School for Wisconsin's primary election Tuesday April 7, 2020, in Milwaukee.

The United States postal service said Friday it is investigating potential issues with absentee ballots in Wisconsin, which held a highly controversial primary on Tuesday in the middle of the coronavirus pandemic.

The probe comes just one day after Wisconsin's US Senators, Republican Ron Johnson and Democrat Tammy Baldwin, sent a letter to the Inspector General of the US Postal Service, urging for an investigation into reports of absentee ballots that were not delivered to voters and the discovery of absentee ballots at a USPS processing center.

The letter cites three issues with Milwaukee County absentee ballots, saying in part that "three tubs of absentee ballots from Appleton and Oshkosh were discovered in USPS's Milwaukee processing center after polls closed on Tuesday" and that batches of absentee ballots requested on March 22 and 23 failed to be delivered to voters.

The third instance cited in the letter involves the village of Fox Point, which is located within Milwaukee County, that reported "that USPS returned to them multiple batches of unsent absentee ballots and they were unable to obtain an explanation from their local post office as to why they were not delivered to the voters who requested them. "

"We are concerned there may be more examples, and request that you promptly open an investigation to determine the cause of these failures, which appear to have disenfranchised many Wisconsin voters," said the senators in the letter.

In a statement on Friday, Postal Service spokesman David Partenheimer said that they "have been made aware of potential issues with absentee ballots in Wisconsin and are currently conducting an investigation into the claims."

"At this time we do not have additional information to provide on this issue," he added.

"It is important to note that throughout every election cycle, the US Postal Service works with state and local election officials to ensure the timely delivery of Election Mail. The United States Mail system serves as a secure, efficient and effective means for citizens and campaigns to participate in the electoral process, and the Postal Service is committed to delivering Election Mail in a timely manner."

On Wednesday the Milwaukee Election Commission called for an investigation into missing ballots. Executive Director Neil Albrecht said they wanted the investigation to center on ballots requested and mailed but that never reached voters, according to the Milwaukee Journal Sentinel.

However, Reid Magney, spokesperson for state officials, the Wisconsin Election Commission, told CNN that they don't "know a whole lot about" the investigation and "are in the process of gathering information."

Thousands of people requested absentee ballots ahead of April 7 election, but local clerks reported to the commission that many of those voters would not receive those ballots in time to mail them back.

More than 1.2 million absentee ballots were sent out for the April 7 election, with just a little over one million returned as of Friday, according to commission.

In the lead up to the election, Democrats battled Republicans in court over extending the absentee voting. In an initial ruling, US District Judge William Conley ruled to extend the deadline to return absentee ballots from April 7 to April 13. He also pushed back the state's deadline to request absentee ballots from a Thursday to Friday. But that lower court ruling was then blocked by the US Supreme Court in a late night ruling on the eve of the Tuesday election. The court said ballots had to be postmarked by April 7 and returned by April 13 -- whereas Conley had not set any deadline for ballots to be postmarked.