Real Madrid stars Antonio Rudiger, Kylian Mbappe, Dani Ceballos and Vinicius Junior are being investigated by UEFA for their conduct after the Champions League win over Atletico Madrid.
The Champions League holders won a fiercely-contested last-16 tie with their capital city rivals on penalties - and in controversial fashion, with Julian Alvarez's penalty ruled out after officials judged he had touched the ball twice.
Rudiger struck the winning penalty and that set up a quarter-final tie against Arsenal.
The Real Madrid quartet are now being looked into over allegations of indecent conduct made during the post-match celebrations.
Television pictures appeared to show Rudiger making a throat-cutting gesture towards the crowd and Mbappe apparently grabbing his crotch.
UEFA has appointed a special inspector, who will work independently, and most of the evidence is expected to be video footage of the alleged incidents.
UEFA is unable to provide information on how long the process will take and possible sanctions.
Why a fine could be more likely than a ban
A fine is thought to be more likely than a ban. In 2019 UEFA fined Cristiano Ronaldo €20,000 (£16,665) for improper conduct. Ronaldo had made an obscene gesture after scoring for Juventus against Atletico Madrid.
Atletico head coach Diego Simeone was also fined €20,000 by UEFA for making the same gesture after his side scored in the first leg at home.
There is no guarantee the case will be dealt with before the first leg with Arsenal at the Emirates Stadium on April 8. The teams then meet again eight days later in Madrid.
