A stampede at a nightclub near the Italian city of Ancona early on Saturday killed six people and injured a further 120, media and firefighters said.
Italian fire officials and ANSA news agency said fans inside the club panicked and ran for the exits after someone sprayed a substance similar to pepper spray.
A teenager told ANSA he discovered that at least one of the emergency exits was locked when he tried to flee.
Twelve of the 35 injured were in serious condition, Capocasa said.
The Italian fire service tweeted: "The cause may have been the dispersal of a stinging substance, the young people fled and trampled over each other".
Carabinieri paramilitary police Col. Cristian Carrozza said the dead teenagers' ages ranged from 14 to 16 and the woman was 39.
Italian high schools are usually open on Saturdays, but schools were closed this weekend for the December 8 national holiday of the Immaculate Conception.
The incident occurred during a concert by an Italian rapper, Sfera Ebbasta, police said. But Salvini said initial investigation "appears to knock down that possibility".
He said that when rescuers arrived, all the doors of the discos were open.
He said he did not have confirmation from survivors that the use of an irritating spray, like pepper spray, had set off the panic. Another eyewitness said a wall inside the club had collapsed in the melee.
Salvini vowed that responsibility would be determined for the "six broken lives - whoever out of nastiness, stupidity or greed transformed an evening of partying into tragedy".