Days after state agents raided its Delaware County office, a national grassroots organization that works mostly for Democrats is facing increasing scrutiny over voter registration efforts in Pennsylvania.

A source with knowledge of the inquiry said Wednesday the probe of FieldWorks LLC is likely to expand to other spots in the state.

At the same time, Delaware County officials are preparing to challenge thousands of voter registration forms submitted by the group, questioning if they were filed on time. A hearing on the matter is scheduled for Friday.

Both developments came after Pennsylvania state police searched FieldWorks' Norwood office last week seeking, among other things, forms that could be used to "construct fraudulent voter registration forms," according to court records.

The Attorney General's Office has declined to discuss its investigation, or say if evidence suggests a plot to cast fraudulent ballots or just attempts by paid canvassers to artificially inflate voter registration numbers.

Officials say it would be difficult for new registrants who used a false name to cast a fraudulent ballot, because such voters must also provide identification that matches registration information.

FieldWorks calls itself a grassroots political firm founded in 2001 to work with "progressive advocacy organizations, federal and state Democratic campaigns, Democratic Party organizations, labor unions, and ballot initiative campaigns." In ads, it recruits hourly workers for field work. It has said only that it is cooperating with investigators in Pennsylvania and has "zero tolerance for fraud."In 2012, FieldWorks' voter registration efforts in Ohio sparked controversy when election officials flagged hundreds of dubious registration forms filed in Hamilton County, including one from a voter registered as "John Adolf Hitler."

Cincinnati police that year also arrested a former Ohio University student working for FieldWorks on charges of forging 22 signatures on a petition drive. Police at the time said FieldWorks played no role in the man's scheme to pad his list.

The dispute now bubbling in Delaware County has less to do with the authenticity of voter registration forms than when they were filed.

On Friday, the Delaware County Voter Registration Commission - a bipartisan board - will hold a hearing to determine if thousands of forms, many from FieldWorks, were submitted after the state's Oct. 11 deadline to register to vote.

According to Delaware County Solicitor Michael Maddren, the county received thousands of voter registration forms from the Pennsylvania Department of State on Oct. 14 and 17 that had no dates or postmarks.

"We don't have any indication that they were timely receipts," Maddren said.

A spokeswoman for the Pennsylvania Department of State said that when the state forwards any registration applications to the county, "we attest to the fact that they were either postmarked or delivered in person by the deadline."

"If they were not timely received here, we would not send them to any county," Wanda Murren, the spokeswoman, said in an email.

A FieldWorks spokesman on Wednesday said the organization is also confident the forms - in packages with tracking numbers - were delivered to the Department of State before the deadline closed.

The questions about the forms extend beyond their timeliness, Maddren says. County election officials who surveyed the forms, he said, also discovered some applications where individuals were registered to multiple addresses or where registrants' Social Security numbers did not match their driver's license numbers.

But Friday's hearing will focus exclusively on the timeliness issue, Maddren said.

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Staff writers Justine McDaniel and Craig R. McCoy contributed to this article.