PORTERDALE, Ga. - According to his resignation letter, former Porterdale City Manager Bob Thomson knew the city had "lost confidence" in his work. 

"Given that fact and the need for city business to continue, I would like to offer my letter of resignation at this time," he wrote. 

Thomson submitted that letter Aug. 5, which was after the Georgia Bureau of Investigations opened a forgery investigation involving Thomson Aug. 1.  

Porterdale City Council voted 4-0 to approve Thomson's resignation letter, effective immediately, during a special called meeting Monday, Aug. 12.

"We had no idea there were any delinquencies," Mayor Arline Chapman said during the meeting.

Council members Linda Finger, Tim Savage, Mike Harper and Lowell Chambers voted to approve the letter. Council member Terry Gray abstained from voting.

Newton County District Attorney Layla Zon requested the GBI conduct a forgery investigation involving the City of Porterdale Wednesday, July 31, according to the GBI face sheet.

"Very generally, the scope of the investigation involves city funds and the authorization of spending of said funds," Zon said.

Thomson allegedly signed Chapman's name on checks, but the GBI has not confirmed this allegation. The GBI can only confirm what has already been stated on the face sheet, according to GBI Office of Public Affairs Director Nelly Miles. 

Staff Writer Caitlin Jett contributed to this report.