Two deaths occurred Saturday and one on Sunday. Miller says the cause of each death remains unknown.
The sheriff's coroner division plans to release names and ages of the deceased Monday afternoon.
The electronic dance music festival was held at an outdoor venue about 50 miles east of Los Angeles.
According to the Coroner's Division of the San Bernardino Sheriff's Department, the deceased have been identified as Derek Lee, 22, of San Francisco, Alyssa Dominguez, 21, of San Diego, and Roxanne Ngo, 22, of Chino Hills.
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A HARD Summer representative released the following statement on the incidents:
“We were deeply saddened to learn about the deaths of three people who attended the festival this weekend. While the causes of the deaths have not yet been determined, we ask everyone to keep them in their prayers. Our sincerest thoughts and condolences are with their family and friends.”
Updated, Aug. 2 10:31 a.m. ET: names of the deceased and HARD statement.