Police say they have arrested two men after a lit flare hit a linesman when it was thrown onto the pitch during Sunday's Tottenham win at Aston Villa in the Premier League.
The Football Association also opened an investigation after assistant referee David Bryan was hit on the back of the neck by the projectile at Villa Park.
The incident happened just after England striker Andros Townsend opened the scoring for the Spurs. The match was interrupted for a few minutes because of the black smoke but Bryan was able to continue.
Tottenham went on to win the match 2-0 after Roberto Soldado added another goal in the second half.
West Midlands Police said the two men are aged 25 and 47 and confirmed the referee was not injured.