Several counties in Pennsylvania have been hit with voter fraud schemes within roughly a week of the Election Day, leaving county officials scrambling to investigate what’s going on.

Some 2,500 voter registrations were flagged in Lancaster County late last month as possibly fraudulent. The discovery came just before the state’s preelection registration deadline.

Officials said two batches of registrations, totaling roughly 2,500 applications, seemed to be fishy. There were inconsistencies in signatures, duplicate handwriting, and illegitimate addresses, driver’s licenses and Social Security numbers.

Ray D’Agostino, vice chairman of the county’s elections board, said in an update Monday that the investigation into the incident found that 57% of the registrations flagged have been verified as valid, 17% were confirmed to be fraudulent and 26% are either incomplete or non-verified.

“Those other two buckets are going to change, quite frankly, based on the continuing investigation,” Mr. D’Agostino said. “It’s a very painstaking process, obviously, to go through those.”

Some 100 miles north in Monroe County, roughly 30 voter registration or mail-in ballot forms were deemed irregular by the Monroe County Board of Elections, District Attorney Mike Mancuso said in a statement last week. He said the forms were separated and referred to his office for investigation.

“Several of the Voter Registration Applications and Mail in Ballot Request forms have been found to be fraudulent as they were not authorized by the persons named as applicants,” he said in the statement on Facebook.

He said a company called “Field and Media Corps,” a subcompany of Fieldcorp, was responsible for submitting some of the questionable forms. Fieldcorp is an Arizona-based organization working in Lancaster, whose CEO is a Democratic official in the state.

“The broader investigation continues with reference to Fieldcorp’s involvement,” he said.

An update from Friday said additional fraudulent applications had been found. The investigation also found that out of 27 forms referred to the D.A.’s office, 21 were mailed by Fieldcorps.

“Out of the ‘Fieldcorps’ submitted applications, 16 are fraudulent,” he wrote. “The specific fraudulent character of these applications involve forged signatures, often with incorrect or incomplete identifying information.”

He said roughly half of the fraudulent applications are connected to employees of the organization, who are listed as assisting in the forms’ completion.

“Concerning the six non-‘Fieldcorps’ applications, two are actually mail-in ballots which were apparently stolen with the perpetrators unsuccessfully attempting to cast the ballots,” he said. “The other three remain under investigation.”

Francisco Heredia, CEO and partner of Field and Media Corps, reportedly said in a statement that the organization is “proud of our work to help expand access to voting through our nonpartisan voter registration program.”

“We have not been contacted by election officials in PA counties and we have no additional information on the alleged problematic registration forms,” the statement said.

Mr. Heredia is also the Democratic vice mayor of Mesa, Arizona.

The website for the organization seems to have since been deleted.

In York County, Pennsylvania, near Lancaster, Field and Media Corps also made a delivery of election forms on behalf of the Everybody Votes Campaign. Everybody Votes is a nonpartisan organization that promotes voter registration.

In a statement, York County Commissioner Julie Wheeler said that if any fraud is discovered, the district attorney will handle an investigation.

State Attorney General Michelle Henry said in a statement last week that “attempts to submit fraudulent voter registration forms in Berks, Lancaster, Monroe, and York counties have been defeated.”

“While we will not be divulging sensitive information about these investigations, we want to clarify that the investigations regard voter registration forms, not ballots,” she said. “These attempts have been thwarted by the safeguards in place in Pennsylvania. We are working every day with our partners to ensure a fair, free, and safe election.”