Liverpool fans threw a flare into the section at Old Trafford during their 4-3 defeat to on Sunday, according to reports.
The quarter-final clash was also marred by tragedy chanting with Greater Manchester Police confirming that an arrest has already been made.
The flare incident occurred when scored in the first half to give Liverpool a 2-1 lead, the has reported.
It was thrown from the section of away fans and fell beneath them – landing in the wheelchair section below – which houses disabled United supporters.
Meanwhile, tragedy chanting aimed at Liverpool fans occurred at various points with United supporters heard singing ‘always the victims, it’s never your fault’ and ‘The Sun was right – you’re murderers’ in reference to the Hillsborough disaster.
‘GMP can confirm that an arrest has been made in connection with tragedy chanting at the Manchester United v Liverpool fixture this afternoon,’ said a spokesperson from Greater Manchester Police.
‘Incidents of tragedy chanting will not be tolerated and will be dealt with robustly. We are all familiar with fans taunting their opponents at football matches, but this is generally done in good humour and without offence.
‘The incidents today crossed the line of what is acceptable, and we will work closely with both clubs to identify anyone responsible for committing offences during the event.’
An FA spokesperson added: ‘We are aware of the reported tragedy chanting during today’s FA Cup fixture between Manchester United and Liverpool and we will be liaising with and supporting Greater Manchester Police.
‘We strongly condemn any offensive, abusive and discriminatory chants in football stadiums, and we are determined to stamp this behaviour out.
‘It is entirely unacceptable and can have a lasting and damaging impact on people and communities within our game.’
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