UEFA is to open an investigation into Benfica fans' behavior at Atletico Madrid on Wednesday evening, according to Sky sources.
Benfica expect to be charged by the governing body following trouble at their Champions League game at the Vicente Calderon Stadium.
Portuguese fans were caught on camera throwing flares into a section of the ground full of Atletico supporters following the 2-1 victory which lifted them to the top of Group C.
Benfica has released a statement apologizing to Atletico, with club president Luis Felipe Vieira admitting it has cast a "shadow of sadness" over the result.
"I have mixed emotions, happiness and disappointment," he said. "Certain things are difficult for us to control.
"Sadly a small group of trouble-makers has tarnished the good name of Benfica, just as the players honoured the playing shirt and made us all proud after the win in Madrid.
"We have won nothing and we now need to focus on the league game against Uniao Madiera but with a shadow of sadness after these incidents."
