Toronto have apologised to Major League Soccer rivals Montreal Impact after a "disgraceful" banner showing a sex act was displayed during Saturday's game.
A club statement has asked fans to help the investigation by identifying who was responsible for the banner, which it described as "an embarrassment".
It added that the sign was "insulting to many, including our opponent".
Impact beat MLS Eastern Conference leaders and Canadian rivals Toronto 1-0 with a 73rd-minute Ignacio Piatti goal.
Montreal supporters reported the banner to Toronto officials on social media, with the club responding that the sign was "not OK by any means".