Chelsea have promised to punish any fans found to have made death threats to referee Terje Hauge in the wake of the Champions League loss to Barcelona.
The threats appeared on messageboards on Chelsea's official website after the Norwegian red-carded Asier del Horno.
A club statement said: "If Chelsea are able to trace the authors, and if they are season-ticket holders or members, then action will be taken."
The club also vowed to conduct a review of monitoring procedures on the site.
The statement said Chelsea "totally condemns" such death threats but warned that tracing the source of the messages may not be easy.
"Chelsea would like to stress that, at this stage, it is impossible to say whether the messages were posted by Chelsea fans as the site is publicly accessible," it added.
"Similarly the club is not in control of other fan websites, inside or outside of the UK that claim connections with Chelsea, and those websites are also publicly accessible."
Hauge gave Chelsea defender Del Horno a straight red card for a challenge on Lionel Messi in the first half of the first leg, last 16 encounter at Stamford Bridge.
The dismissal eventually cost Chelsea as they slipped to a 2-1 defeat.
The controversy follows the threats made to Anders Frisk after the two clubs met at the same stage last season, prompting the Swede to retire.
Last year, Frisk sent off Blues striker Didier Drogba in the first leg at the Nou Camp.