HARRISBURG, Pa. — A criminal investigation is underway in crucial presidential battleground Pennsylvania after election officials in one county flagged about 2,500 voter registration forms for potential fraud. Two other counties were alerted to look for similar problems.

The forms arrived at the Lancaster County elections office shortly before the state’s registration deadline last Monday and were apparently part of a larger effort to register people, officials said Friday. Some had fake names, suspicious handwriting, questionable signatures, incorrect addresses or other problematic details.

Two other counties “received similar requests” and were asked to review them, Lancaster County Prosecutor Heather Adams said at a news conference. She declined to name the other counties.

“It appears to be an organized effort at this point,” said Adams, a Republican lawmaker. “But of course, it’s an ongoing investigation.” And we will see who exactly participated in it and how far it goes.”

The pool of applications was separated during the investigation, preventing ballots from being sent or cast until they were deemed valid. Pennsylvania’s 19 electoral votes are being contested as polls indicate the presidential race in the state is a close race.

“The fact is we contained this,” Lancaster Commissioner Ray D’Agostino, a Republican who chairs the elections board, said at the news conference. “It’s not good. It’s illegal. It’s immoral. And we found it, and we’re going to take care of it.

Adams and members of the county election board did not say who filed the forms or who they may have worked with.

D’Agostino said the applications were not limited to a single party and were collected from various locations in predominantly Republican Lancaster.

The Pennsylvania Department of State released a statement Friday praising election officials “for their diligent work in spotting this potential fraud and bringing it to the attention of law enforcement.” The state attorney general’s office, which was also contacted by Lancaster officials, declined to comment.

About three out of five apps that have been thoroughly investigated so far have encountered problems, Adams said. Other requests among the 2,500 have been verified as accurate and are being processed as normal, she said.

Most of the applications were dated August 15 and were predominantly from Lancaster City. Adams said the applications were collected as part of a “large-scale canvassing operation.”

“In some cases, the applications contained correct personal identifying information, such as correct address, correct phone number, date of birth, driver’s license number, and social security number – but people listed on the applications informed detectives that they had not requested the form,” Adams said. “They did not complete the form and verified that the signature on the form was not theirs.”