Manchester United have launched an investigation after two fans slept at Old Trafford the night before Saturday's clash with Arsenal.
The two fans were on an official tour of the stadium on Friday and hid in the toilets.
They were found before the doors were opened ahead of the game, during a routine pre-match search, and handed over to the police, but were later released without charge.
A Manchester United spokesperson said: "We realise people will go to any lengths to watch a game at Old Trafford but these people went too far.
"They had already been searched and scanned before the tour, so we are confident there was no problem for safety."
The two fans did not get into the game, which United and Arsenal drew 1-1 thanks to Olivier Giroud's late equaliser for the away side.
Manchester United's next three fixtures are all at Old Trafford, with Thursday's visit of Feyenoord in the Europa League followed by a double header against West Ham, first in the Premier League and then in the EFL Cup on November 30, live on Sky Sports 1.
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