In addition to the two people in the apartment at the time of the attack, two police officers and seven emergency workers also sought treatment for exposure to the substance, which authorities initially suspected to be hydrofluoric acid, according to Sveriges Television (SVT).
Initial reports had put the number of victims at 13, but the figure was adjusted downward to 11 later in the day on Wednesday.
Police are also less certain about the exact nature of the substance, admitting that they have no idea what the substance could be.
“They’re on their way here from emergency services in Gothenburg to do a quick test,” police inspector Lennart Wennblom told the TT news agency at lunchtime on Wednesday.
The attack took place around 2am in a the Öxnehaga residential area of Husqvarna outside of Jönköping.
According to police, five windows were broken and and the liquid was thrown into the apartment, but exactly how remains unclear.
The two people in the apartment at the time, both in their twenties, were awakened by the sound of breaking glass.
“When police and emergency services arrived on the scene they said it was a brownish liquid which immediately caused severe throat irritation,” said Jönköping police spokesperson Nils-Erik Eriksson to the TT news agency.
Everyone present in the apartment has been admitted to hospital for observation for the next 48 hours.
According to Wennblom, all the victims feel well and police continue to work on uncovering a motive for the attack.
Hydrofluoric acid is highly corrosive and can quickly penetrate skin, sometimes weakening bones underneath. It can also interfere with nerve functioning, sometimes delaying awareness of any developing chemical burns.
If it penetrates the blood stream, hydrofluoric acid can also cause cardiac arrest.