Southampton - Manchester United reiterated their condemnation of a crude chant about Romelu Lukaku after it was aired again during Saturday's 1-0 Premier League win at Southampton.
Both United and Belgian striker Lukaku had previously implored supporters to stop singing the song, which draws on racial stereotypes about the size of black men's penises.
But fans sang it again at St Mary's, as well as "We're Man United, we'll sing what we want", and the club now plan to use CCTV footage to identity the offenders.
"Manchester United has a zero-tolerance stance on offensive chanting and behaviour," United said in a statement.
"The club and the player have been clear in asking for an end to the chant.
"The club has worked with relevant bodies and supporters groups in trying to eradicate any offensive behaviour and will take further action against individuals if this continues.
"The club is in discussion with the police and has asked for CCTV footage from Southampton and will try to identify those who disrespected the player's wishes not to sing the song."
Lukaku, 24, scored United's winner at Southampton, taking his tally to eight goals in eight games since his £75 million transfer from Everton.
Earlier this week, Lukaku released a statement saying it was time for the club's fans to "move on" from the chant, which has also been criticised by anti-discrimination groups.