(CNN) Law enforcement officials in Delaware County, Pennsylvania, are looking into the authenticity of a number of voter registration forms submitted by FieldWorks, a political grassroots organization.

Delaware County received 6,300 voter registration forms in two shipments from the Pennsylvania Department of State, according to Delaware County Solicitor Mike Maddren. The forms were submitted to the state by FieldWorks.

The county got the shipments on October 14 and October 17. The state requires such voter registration forms to be submitted by October 11. It's unclear how many of these forms got to the state by this time.

On November 4, the Delaware County Voter Registration Commission held a hearing to determine whether or not the voter registration forms the county received after the deadline could be considered valid. Delaware County Department of Voter Registration Chief Clerk Mary Jo Headley informed the commission that 3,840 of the forms were invalid for various reasons, including duplicate registration, invalid addresses and other discrepancies.

FieldWorks' county office searched

Headley found 1,160 voter registration forms to be valid. The commission determined that if any of those valid voters show up to the polls on election day, they will have to vote with a provisional ballot.

Ballots will be counted Thursday after the election and can be contested while they are being counted.

Delaware County officials executed a warrant to search FieldWorks' county satellite office, according to court documents.

County officials cited possible "tampering with public records or information" as the reason for the search. Police noted they would be searching material "relating to identifying co-conspirators," and "any/all voter registration forms, including but not limited to ... any templates (physical or electronic) utilized to construct fraudulent voter registration forms."

Police went to the office the night of October 28, according to FieldWorks spokesperson Matt Dorf.

Grassroots group 'working closely' with county

"The integrity of our electoral system is a foundation of our democracy, which is why FieldWorks has the most rigorous quality controls in the industry and zero tolerance for fraud," FieldWorks said in a statement to CNN.

"FieldWorks is now working closely with county officials to provide them with information on our program and applications they are investigating."

FieldWorks is a national political grassroots organization that has worked with a number of Democratic clients in the past. It provides canvass operation services for campaigns by organizing door knockings, phone banks, signature collections and voter registration.

FieldWorks sent the voter registration forms to the Pennsylvania Department of State, but when it sent the forms, before or after the deadline, is unclear.

"When you look at these, they have Department of State stamps on them that are after the deadline," Maddren said. He stated that the majority of the forms were stamped after October 11, but some of the forms do have dates before the deadline as well.

It was unclear whether the forms were received before the deadline and then stockpiled by the department, or whether they were received and sent late, according to Maddren.

'We run good elections'

In the Friday hearing, the Department of State told the Delaware County Voter Registration Commission that the voter registration forms were received before or on the deadline even though Delaware County officials received some of the forms late. During the hearing, commissioners referenced a total of 5,000 voter registration forms received late. It was unclear why that figure differed from the 6,130 number mentioned earlier by Maddren.

Both the Pennsylvania Department of State and the Pennsylvania attorney general's office would not comment on the ongoing investigation.

In a press conference on October 20, Secretary of State Pedro Cortes reaffirmed his confidence in the Pennsylvania voting system, despite recent campaign rhetoric.

"Some have suggested that fraud is rampant and election officials at the local and state level have ill intended motives. This is not only wrong and uninformed, it is also dangerous," Cortes said. "I delivered this message in 2004 and 2008. We will not give you much to write about. We run good elections."

The Delaware County Voter Registration Commission will hold a hearing on Friday focusing on the timeline in which the voter registration forms were received. The hearing will not cover the authenticity of the documents themselves.