Paul White, AP
The venue's capacity of 12,000 was not reached, according to a spokesman for the arena, but several attendees on Twitter estimated the space held 20,000 and that the concert had become overloaded.
By 4AM, medical teams were dispatched to the venue after a stampede broke out. Spain's El Pais reports that people suffered "trauma by crushing, possibly having suffered a fall and being trampled."
EMTs tried to resuscitate two victims but declared them dead after 30 minutes. A third person died from their injuries at a local hospital.
An investigation has been opened, but police believe a flare was set off in the crowd, creating the panic that led to a full-on stampede. The fatalities were not caused on the dance floor, but in a narrow hallway below the venue.
Witnesses report concertgoers were lying on the ground, being stepped on as people ran to exit the building. Police did not completely clear out the venue until three hours after the crowd crush occurred.
"After my performance last night at Thriller Music Park at the Madrid Arena in Spain, I was devastated to hear that three people had died during a crowd rush at one of the exits," Steve Aoki said, in a prepared statement.
"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 continued. "My show in Bilbao, Spain tonight will be cancelled out of respect for those who lost their lives and those who are injured. I would like to express my deepest condolences and heart felt prayers to the families and loved ones during this difficult time."