South Sydney have been unable to identify the fan who threw a metal socket at Manly centre Steve Matai during their NRL match at ANZ Stadium on Sunday.

The club have declared their investigation into the incident closed after interviews and studies of CCTV footage proved inconclusive.

Matai was hit with the small object as he made his way from the field having been sin-binned late in the first half of his the Sea Eagles' shock loss.

Souths chief executive Shane Richardson had vowed to ban the offending fan for life but said all avenues of the five-day investigation had been exhausted.

"We have examined everything down to the smallest of details, we have worked with ANZ Stadium and NSW Police, we have spoken with Manly, we have spoken with our members, and unfortunately we have been unable to clearly identify the perpetrator," Richardson said.

"We have done everything we can, and unless someone steps forward and owns up to this, or we receive further information to help with the enquiry, then we consider the investigation to be closed."

Richardson made a final plea to any members or fans who may have seen the incident to come forward.

"Somebody out there knows who this cowardly person is and it's only a matter of time before we know who it is as well," Richardson said.

"This is a serious incident, which goes against everything our club stands for, and it is poor form that a member or fan would do something like this in the first instance, and secondly, tar the name of the club and its members by not owning up to their mistake."

Richardson said there would be extra security at Sunday's game against the Canberra while there would also be fixed cameras on all seating areas.

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