(CNN) -- Three young women are dead and two are in serious condition after being crushed by a crowd at a Halloween party in Madrid early Thursday, police said.
The women were among thousands of attendees at "Thriller Music Park," an event at Madrid Arena that featured electronic music artists and a costume contest.
There was a stampede in an arena hallway that connected the main dance floor to another area, police said.
The five women, between 18 and 25 years old, were the only ones hurt in the incident, an ambulance spokesman said.
Four of the women, including the three who died, are Spanish. The nationality of the other injured woman is unknown.
Police are investigating whether someone set off fireworks in the hallway, the spokesman said.
Thousands of partygoers were unaware of the stampede until after police arrived and began moving people out slowly.
American electronic musician Steve Aoki was headlining the party.
Aoki released a statement Thursday saying he was "devastated" by the deaths and injuries.
"The safety of my fans has always been my top priority and had I known that there was any distress putting people in jeopardy within the venue I would have ended my show immediately," Aoki said in the statement, adding that he was canceling a subsequent show in Bilbao, Spain, "out of respect for those who lost their lives and those who are injured."