A high-profile football match between Argentine rivals Boca Juniors and River Plate was stopped when visiting players were sprayed by fans with an eye irritant believed to be pepper spray or mace.
Buenos Aires public prosecutor Martin Ocampo said Boca's La Bombonera stadium has been closed so investigators can gather evidence.
The Copa Libertadores match kicked off in front of about 50,000 fans. River won the first leg 1-0 last week, and last night's return leg was 0-0 when it was suspended before the second half could begin.
The suspension and fan attack on rival players is the latest in endemic violence that mars many matches in Argentina.
The match turned to chaos when River players, entering the field for the second half, came out of a temporary tunnel rubbing their eyes and pulling their shirts over their faces. Many poured water over their faces to clear their eyes. Police and officials waited on the field for more than an hour before suspending the game.
"I can't say that they (Boca) lost control with the scant information I have," Mr Ocampo said.
But Sergio Berni, a senior Argentine national security official, put the blame squarely on the club.
"Boca is the only one responsible, this is undeniable," he told reporters. "The club neglected to provide internal security."
River and Boca players milled around at the centre of the field after the match was called off, apparently fearing for their safety because many fans were trying to wait out attempts to clear the stadium.
River said in a medical statement that at least four players were diagnosed with inflamed corneas, and several were burned from the spray.
"My whole body hurts," River player Leonardo Ponzio said. "They threw - I'm not sure what it was - pepper spray at us. This just can't be."
Television images showed a fan sliding a hand through the makeshift tunnel that shields players entering the field.
Argentina already bans visiting fans from matches, but that has not stemmed the violence as rival hooligans gangs affiliated with the same club often battle over turf, rights to park cars and reselling tickets.
Nestor Benitez, a spokesman for the governing body of South American football, said officials were awaiting a report from the referee before saying how the suspended game would be handled.
This was the third match in 12 days for the two Buenos Aires rivals. Boca defeated River 2-0 to take the lead in the Argentine first division, just ahead of River. Four days later, River defeated Boca 1-0 in the first leg of the round of 16 at River's Monumental stadium.