Four Boca Juniors fans suspected of spraying toxic gas at River Plate players during the Copa Libertadores were formally charged Tuesday with aggravated assault, a judicial source said.

The four men allegedly involved in the May 14 attack -- in which a pepper-spray-like substance was fired at several River Plate players -- were released after posting a bond equivalent to $10,700 each, according to the ruling by Judge Wilma Lopez.

The four, the first to be charged over the unsavory incident, were expelled from the Boca fan club in July.

Judge Lopez declined to bring charges against four other suspects, citing a lack of evidence, according to the Judicial Information Center.

River Plate players were returning for the second half when a fan or fans apparently released a chemical irritant into the inflatable tunnel leading from the dressing rooms to the pitch.

The attack left several players rubbing their eyes in agony as teammates desperately threw water in their faces to try to relieve the pain.

Boca were disqualified from the Copa Libertadores after the incident in the round-of-16 match.

The South American football federation CONMEBOL fined the team $200,000 and forced it to play two games this season in front of empty stands at its Bombonera stadium.