A criminal investigation is underway in the crucial presidential battleground of Pennsylvania after election workers 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 deadline to register this past Monday and were apparently part of a larger effort to sign people up, officials said Friday. Some had false names, suspicious handwriting, questionable signatures, incorrect addresses or other problematic details.

Two other counties “received similar applications” and were notified to check into them, Lancaster County District Attorney 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, an elected Republican. “But of course, it’s an ongoing investigation. And we’ll be looking into who exactly participated in it and how far up it goes.”