The second person was interviewed by the FBI before being cleared by it.
"The unidentified subject has been located interviewed and does not appear to be connected with the yesterday's shooting incident at Oak Creek," a FBI spokesman told PTI.
Even as the investigation is ongoing, the FBI said the shooting was carried out by the lone gunman identified, as Wade Michael Page, who was later shot dead by an Oak Creek police officer.
Earlier on Monday at a news conference, the FBI said they were trying to identify a suspicious man, who arrived at the scene after the shooting and released a photograph of him, asking for the public's help.
The investigation is still being treated as a possible case of domestic terrorism, the local police said.
On August 5, six persons were killed and three critically injured when a gunman opened fire inside a gurudwara in Wisconsin.