Pompey are braced to receive a fine following Saturday’s crowd disturbance.
A number of home supporters broke out of the North Stand to confront Oldham supporters towards the end of the 2-1 defeat.
In doing so, they encroached on the pitch – forcing referee Craig Hicks to temporarily halt the match.
The Surrey-based official is now expected to include the incident in his report to the Football Association.
I thought our security did very, very well holding back the majority of supporters and thankfully there were only some minor injuries as a resultMark Catlin
After the final whistle, with Blues fans in Specks Lane behind the Milton End, Oldham followers were diverted along the touchline of the North Stand to an exit in the corner of the Fratton End.
Chief executive Mark Catlin insists the club are investigating the issues.
And he believes Pompey could be fined.
Catlin said: ‘We are working closely with our security team and the police looking into any possible causes and, ultimately, if anyone broke ground safety rules or the law.
‘We could definitely be liable for a fine. The figures are down to the FA, so I cannot speculate.
‘Thankfully it’s unusual in this day and age to have pitch encroachments like that.
‘Obviously there are isolated incidents, but to have those numbers involved is rare.
‘I thought our security did very, very well holding back the majority of supporters and thankfully there were only some minor injuries as a result.
‘The referee stopped the game as a result, so I am assuming it will be mentioned in the report.
‘Then the FA will come to us seeking our observations.
‘In the meantime, we are working behind the scenes conducting a full and thorough investigation.’
– NEIL ALLEN